Without going into too much detail, the importance of light level is explained simply by damage differences. The lower level you are from the enemies you are fighting, the less damage you will do and the more damage you will take. This results in more deaths, and less firepower. As no real numbers have actually been released, test have been conducted by members of the community and have found roughly these numbers:

It also appears that the damage doesn’t scale upwards, that being you don’t do more damage to enemies lower level than you. So when raiding, it’s important to keep in mind what you bring to the table in terms of advantages, or worse, disadvantages to a group.
 

The first thing a group needs to do when starting the raid is build the bridge. This will get you used to a mechanic that is used two other times during the raid. The bridge begins to build once someone steps onto the platform. For this bridge and the bridge at the end of The Abyss, it is not necessary to remain on the platform once the process has started. However, during the actual bridge encounter, someone needs to be on the platform at all times, or the bridge will deconstruct, but we will cover that in the Bridge event section.
Once the bridge is completed, you cross and then drop down a hole in the middle of the Hellmouth. It is a long drop, and it does slow you down briefly to make sure that you don’t take 100% damage from the fall. However, landing on someone can cause you to die, or be thrown in a random direction and actually off the ledge behind and to the right of the platform you land on. While this isn’t a big issue on normal, someone dying on Hard usually prompts the group to wipe and respawn with everyone at the bottom.
 

 

The Abyss is an event as there is no real fight to be concerned about and can actually be done without killing a single mob, but that is quite difficult. The main mechanic of this area is Weight of Darkness, which slows your character’s movement and prevents double jumps. The debuff stacks up to ten times and the effects become stronger the higher it stacks. Because of this, changing your attributes to favor agility is often recommended. Certain weapons also have abilities that allow for more agility or faster movement when equpped (ie. Mida Multi-Tool), however, that is mostly preference. Because not everyone will have the same agility, or the same equipment, the group will usually move at a moderate to fast past. Simply sprinting your way to the end and leaving your group behind is definitely not recommended unless it is what you have been instructed to do. The area is also dotted with pits that are instant death for anyone who falls into them. They are usually easy to spot, but a few of them are in tricky locations, such as behind a few stones that people are tempted to jump over, or at the top of hills just as the path crests. If you are unsure where these holes are, it is best to follow someone who does.
Along the path are a series of lanterns which, when stood next to, removes one stack of Weight of Darkness per second. These lanterns are activated and only emit light for a short time before they turn red and finally explode. You don’t want to be in the area when they explode as you can die, or be thrown off the path and lose your way. This is also why it is important for a group to stick together, that way everyone has roughly the same stacks of Weight of Darkness and requires the same amount of time under the lantern. If someone is running ahead and activating the lanterns by themselves, the people left behind will have less time to use them when they arrive.
During the whole run, thralls and cursed thralls will spawn in. Although, after the first two lanterns, the number of cursed thrall you encounter is significantly less. It’s important to watch for the cursed thrall as they can severely damage someone or even killing them, requiring the group to either go back, or leave them behind. While leaving someone behind might seem like a viable option, the advantage to bringing them back is that when you are revived, you have no stacks of Weight of Darkness. Unfortunately, the person who revived you will have more stacks having to run back to do so. Sunsinger Warlocks with Fireborn can bring themselves back. This is even more useful in Hard Mode as there is no reviving.
At two points in the run, a knight will spawn. A well-equipped group should be able to handle them, however, in Hard Mode, they are Hallowed Knights and are far more difficult to deal with. On top of that, they will almost surely one-shot anyone who gets hit by them, so the best solution on Hard is often to disable them or slow them down. A Striker Titan can hit them with Flashbang Grenade to blind them and leave them wandering aimlessly long enough for the group to move by. Another solution is to have a Defender Titan with Helm of Saint-14 drop a Ward of Dawn behind the group so enemies running through it will be blinded. Lastly, should you have none of these options available, the Knight’s reaction to being stuck with, or be near, a grenade is to huddle down and wait for the explosion. While this isn’t as effective as blinding them, it does buy a few seconds to escape should he get close to the group.
The last part of the event is the bridge, which needs to be built the same way as the entrance to the raid. Once the platform has been activated, the bridge will start to form and doesn’t require someone be on it at all times. Starting this section also starts several waves of enemies, most of which will be thralls and cursed thralls. However, from time to time groups of knights will spawn and rush whoever is alive. Roughly half way through the process, ogres will spawn near the second last lantern and move towards the group. There are three common ways to deal with this event.

Once the bridge has been constructed, a notice will appear saying “The Path Forward is Clear”, and all stacks of Weight of Darkness will be lifted. At this point, the group just needs to cross the bridge and head towards the light. There will be an ogre that spawns in on the other side of the bridge, but is easily avoidable. Reach the light and the group will receive rewards and be teleported to the next area.

 

The second event of the raid is the bridge encounter, which has a few mechanics of it’s own. The first thing you will notice is that the sides of the map are fairly symmetrical, being that there are the same locations on both sides. This will come into play with the mechanics. The goal of this encounter is to get as many people across the bridge as possible before fighting waves of enemies ending in 2 ogres.
The first mechanic of this area is the bridge and the totems. You will be familiar with the bridge mechanic from previous areas in the raid, but in this area, someone is required to be on the platform at all times for the bridge to complete and remain completed. However, as long as someone is on the platform, the two totems on either side of it will start to glow unless there are people standing in their light as well. If one of the totems is left too long, it will wipe the entire group. This means that three people are required at least to keep the bridge up.
As soon as someone steps on the bridge platform, the event will start and enemies will begin to spawn. The bridge will also begin to construct, and is faster than the previous two bridges. A few seconds after the event starts, a red knight called the ‘Swordbearer’ emerges from the door in the middle of the staircase and heads towards the group. It’s usually random which direction he goes, but will usually go after the person doing the most damage to him. Once he is killed, he will drop his sword. The sword is the same one that is used in the Sword of Crota mission on the moon, or the ones dropped by the minions of crota world events on earth and the moon. The first person to cross needs to get the sword and cross the bridge. You can only cross the bridge while holding the sword.
On the opposite side of the bridge, when the Swordbearer is killed, a blue knight called the ‘Gatekeeper’ will spawn and begin to move around. He can only be killed with the sword. The person with the sword needs to kill the Gatekeeper before the sword disappears, otherwise the next person across will have to deal with him and a new Gatekeeper. The Gatekeeper can be forced to path off the cliff with grenades by clever warlocks, but it’s best to just kill him with the sword. On normal mode, a level 30 can kill the Gatekeeper with three heavy slams of the sword. On hard mode, the Gatekeeper requires 5 heavy slams from a level 32. It is possible to kill the Gatekeeper with the heavy slams and take no damage as it will stagger him, however, timing is important. Simply spamming the slam button isn’t as effective as timing your swings.

Once the sword disappears and the Gatekeeper has been killed, that person must stay alive and fight off enemies spawning in until two more people cross the bridge. Once three have crossed, they can take over building the bridge and standing under the totems. Standing on the bridge platform before there are two people to stand under the totems will cause them to glow and wipe the group. As soon as the group on the opposite side of the bridge are in place, the group on the first side can then move to higher ground and work on getting themselves across. Standing on a pillar or a rock is helpful as it will keep you free from the thralls, however, acolytes and knights can still shoot you.
The next part of the event starts either when everyone has crossed or there is at least one person across and everyone on the other side is dead. Since the waves of enemies include knights, wizards, cursed thralls, and eventually ogres, it’s important to have as many people across as possible. A Sunsinger warlock with Fireborn can bring themselves back after the enemies on the first side have despawned and can assist by shooting across with a scout rifle or a sniper rifle, but this is considered a minor cheese. When the waves start spawning, it’s no longer important to hold the bridge up, or stand under the totems, and is recommended the group get to higher ground to avoid having to deal with thrall or cursed thrall. After the wizards spawn, ogres will be the last wave, one from each of the doors opposite the totems. When the ogres are killed, the event is over and the group is rewarded. The bridge will build automatically after a few seconds for those caught on the other side to move across.

 

The next event is fairly quick and straight forward, but there is an optional challenge that will land the group the second chest in the raid. The second chest is completely optional and will not prevent the group from moving forward.
The event starts when the group enters the hallway, when thrall and cursed thrall will begin spawning and rushing the group. There are two invisible barriers, each with a Shrieker in front of them. The Shrieker needs to be killed for the barrier to drop. Once the second barrier is down and someone makes it into the room with the hole in the floor, the checkpoint is reached and the second chest is no longer reachable if someone hasn’t made it into the room.
 


 

The Deathsinger encounter is a timed event, so it must be done quickly. Once the notice “Deathsinger prepares her song”, the group has 3 minutes to finish the event, so time is very important. This is also where having heavy ammo synthesis comes in handy as you will want to put as much damage on the deathsinger as possible in a short amount of time. When there is only 30 seconds remaining, the group will get a warning saying, “Deathsinger has started her Death Liturgy,” and it will count down. Once it reaches zero, the group wipes.

Killing Ir Yût is the goal, however, she is in a room surrounded by barriers similar to the ones in the Shrieker Hallway. The difference is the Shrikers don’t spawn in until you kill the two wizards flanking Ir Yût in the room. You will need to draw them out, and the easiest way to do this is to enter the rooms on either side of the room Ir Yût is in, and fire a couple rounds at the barrier in front of them. This will cause them to move out and into the open. Once you kill a wizard on either side, the Shriker on that side will spawn and you will be able to take it out. Both Shrikers need to be taken out for the barriers to drop.
As there are quite a few knights and acolytes that spawn in during this event, the safest place for the group to set up is on the platform where Crota spawns in. This is identified by all the rubble on the platform on the opposite side of the map that you enter from. Once someone draws out the wizards, the group can take them out from a distance. The wizards have Solar shields, so both Ice Breaker and Black Hammer are incredibly effective. As soon as the wizards are dead, someone will go back into the rooms on the sides of the deathsinger’s room and destroy the Shriekers to lower the barriers. When the Shrikers are killed, you should leave the room briefly to avoid being killed by the tracers that follow.
Once both Shriekers are down, the barrier is dropped and the group can put damage on Ir Yût. She will circle the summoning crystal in the middle of the room and not leave that room. If she starts her Death Liturgy, she will remain in place until it is finished or until she is killed. As there are still quite a few knights, thralls, and acolytes spawning in during this time, there are two common ways of completing this phase.

If you manage to kill Ir Yût in time, the event doesn’t end for a few more minutes. It is best to use this time to clear out the remaining enemies in the area as you don’t want any left alive before you begin the next, and final, encounter. Once the event ends, the group is rewarded.
 

The final encounter of the raid is Crota, which is also on a timer. At twelve minutes, if Crota isn’t dead, he will enrage and summon the oversoul in rapid succession, as well as summon ogres constantly. It is almost a guaranteed wipe if he isn’t close to death already. This fight has several phases, so we will be dealing with them one by one.

 

Abyss Map: http://www.destinygamewiki.com/images/2/28/Crotas_end_abyss_map1.png
Crota’s End Loot Table: http://destinytracker.com/destiny/crotaloot
Light Level Requirements: http://destinytracker.com/Forums/Post/4227/2/how-much-light-is-needed-per-level