Lord Asag is a custom notorious monster found in Meriphataud Mountains at H-11.
What are the rewards?
Completing the quest will reward you with 2 eggs and a trick staff II. The eggs are used for gear Augments and gambling at the table next to Joseph in Lower Jueno. You will always get a V egg coupled with a random egg from this list: R, G, H, V, J, and D. You can read more about these eggs and what they do here.
How do I start/finish the quest?
Start the quest by talking to Venika in Lower Jueno. After killing Lord Asag, return to Venika to receive your reward. This quest is repeatable, you have to talk to Venika before killing him each time. If you want some of the halloween items like the Trick Staff II and the Pumpkin Head as well as R eggs used for the gambling table, you can also trade a bloody robe and a magicked skull to him for these rewards.
How do I kill him?
So Lord Asag is certainly no pushover. I recommend a solid group of at least 12 characters for this fight split into a party with a tank and support and another party with damage dealers. He has a very nasty AoE attack that also stuns that he likes to spam. For that reason, you want to avoid melee dps and use ranged dps for the fight. Rangers, Summoners, and Black mages are the damage dealers of choice for this fight. Be aware that if you are using black mages, Lord Asag has about a 50% resistance to magic so your nukes will do much less damage than you are used to. Coronach, Namas Arrow, and Predator Claws from Garuda seem to be the most consistent damage for this fight. As far as tanking the fight is concerned, Lord Asag also has a single target tp move that can do 600+ damage to even the most well geared paladins. I recommend your best geard pld/nin tank for this fight with a rdm, brd, and a dedicated healer(whm or sch) supporting him. Using this strategy with enough damage dealers should allow you to take him down relatively quickly so you can do the quest again. His respawn timer seems fairly short so you shouldn't have to wait too long for him to respawn.