He is a large Skeleton with only the upper half as his body.
Yagluth is summoned by obtaining five fuling totem’s and placing them at the sacrificial altar in a seed-determined specific Plains biome. There are stones that give the location of the altar.
The alter can be found within four giant stone pillars. Within the pillars is a small altar that has five small circles for which you can place the totems.
There is a red tablet in front of the alter that can be used once the totems are placed to complete the sacrifice, once done the boss will be summoned.