Appears throughout most of the later rounds. Looks like a giant Shadow Blast. Explodes on contact with the El Crystal dealing around 10,000 damage (enough to kill most players at full health). After the Hunting Fields patch it has had its HP increased from ~15k to ~50k.
After completing the dungeon, you will receive the Elrios Guardian Hero buff. This buff will increase your EXP gain, Additional Damage, and Reduced Damage stats by a certain percentage. The level of this buff is determined by the number of El Crystals protected by the entire server. The buff will increase your stats starting when you finish the dungeon. The buff only applies to PvE play and not PvP matches. The buff is immensely helpful for running Henir's Time and Space Challenge Mode and can drastically cut down run times.
After the Hunting Fields update, the buff will last until GoD is over. If the buff is gained after GoD is completed it will remain for several hours but it will have significantly reduced stats, this is because the buff it gives is proportional to the current progress in defending the crystals which defaults to 0 when the event is not active.
On Very Hard, it is possible to go into the portal and clog up the monsters. They will continue to come out as planned but stay in the portal with you making them easier to kill. You will be pushed out of the portal by shielded mobs and Darkness. This strategy is only effective depending on the class. Female classes have the easiest time getting in and out of the portal and can take out shielded mobs fast enough to keep more mobs from getting through.
It's recommended for Male classes to take the bottom row when present while Females clog up the top portal(you only need 1 top portal camping with someone picking up the drops below)
These strategies are no longer recommended after the Hunting Fields patch as drops are no longer shared and each player must pick up their own drops individually.