Omni Arcanum, or OA, is a monthly vortex event where you can get elgifs, zel, karma, decent exp, and more. You can think of this as a combination of Super Karma Dungeon, Jewel Parade, and the retired dungeon, Forbidden Tablets. You also get special tokens everytime you clear, you can redeem these tokens in the Event bazaar for additional goodies like fusion materials, evolution material, shards, tomes, and golems and more! Sometimes you get powerful elgifs like the Brave X series dropping. OA is always worth your time.
Omni Arcanum is not a hard dungeon so bring any squad you want.
Vortex is home for a lot of event based dungeons so always remember to check on it when you can reap some unique and useful rewards.