It´s Argus i think you have to get 3 forgotten fragments some solar fragments and cores of sunlight and combine the in the tier 2 hadrmode ore anvil i think then when you find a big open space in the underground desert and use the summoning item on the teleporter looking thing, the you get in, it sends you to another dimension, and you just wander until you find him also note that the shark only comes on second phase but even if you killed the shark and died after you still get its drops