Korea and Thailand
This is a field boss event that took place in the Infinite Hunting Grounds (Segarro Coast and Misteye Highlands zones) which required a Greed or Keltaz (Kertas) sealed scroll (from non-archdevil dungeon drops) that was placed into a Dark Truth Brazier to initiate the process of summoning the boss.
A system wide announcement would be displayed informing players that a field boss had been summoned and is attacking the zone. Any player that entered the circular radius surrounding the boss (seen on the mini/world map) would be automatically dropped from their party (if in one) and transformed into their Devilian form where it also became a PVP zone (guild members and guild alliance members would remain as allies). While players could leave the zone (returning to normal form), their contribution/participation would be reset if they stayed out longer than 15 seconds.
When at least one player has entered the summoning circle, a 15 minute (900 second) countdown begins (the main boss spawns once this expires) and monsters/mini-bosses are spawned in 3 minute increments. The countdown stops if all players leave the summoning circle zone.
The reward structure remained the same as regular co-op field bosses where this is both a participation reward and contribution reward. Everyone who participated usually would get some token prize that included items like health potions as well as some minor experience, gold, and devil souls. The better reward was the 2nd box given to the top contributors. This was awarded as follows.
Participants ----- 2nd box reward (Chaos Treasure Box)
1-3 Everyone
4-5 The top 2 contributors
6-10 The top 3 contributors
11-20 The top 5 contributors
>20 Anyone who contributed 20% or more damage
Players could steal contribution from non-allies during the battle as well as steal the top spot if they got the last hit on the boss. There were actual strategies for stealing contribution, resetting the boss (to deny the opposition from last minute sniping), and methods to increase the chances of getting the last hit.
Trion Worlds (North America and Europe)
This was renamed to Devil Hunt with some changes. One of the main differences from the Korean and Thai version is that the boss spawns automatically supplanting the need for an unsealing scroll (the level 43 learning quest and braziers in the Segarro and Misteye zones are irrelevant).
Another change that was made was the removal (in the Fury of the Tempest patch) of the chest reward (Chaos Imbued Chest) that was associated with the last hit mechanic. The player name is still announced system wide as having defeated the boss but no longer holds any real meaning with the removal of the reward since the player does not steal the top contributor spot either (also removed).
Two Trion specific Devilian skill runes (Level 3 or 4 Carnage and more specifically, Bleed rune) are instrumental in increasing the ability to get the last hit. These two runes are extremely rare and were added as part of Trion's monetization design when Enigma Boxes (blue rares) were first introduced. Enigma Keys were initially only available on the marketplace (they were eventually made available via other means including the Archgemstone shop). Level 3 and 4 runes are already random and rare rewards from the box; both level 3/4 Bleed and Carnage runes had the lowest roll percentage and required opening hundreds of boxes (or getting a lot of Chaos chests from Devil Hunts).