So far what I earnt about the creature:

- Find one in the wild, kill it and it drops an altar.(Must be night)

-Place the altar and start summoning.(Must be night)

-To summon you need to have the chosen tributes.

-If you dont have that tribute then wait until the countdown ends and it will still summon the dreadmare AND that creature that should have been tributed.(For me it was alpha deathworm so be careful)

-If you succesfully kill them then you pass but the altar gets damaged because you did not have tribute.(Total hp is 5000 for the altar) When missing 1 tribute it gets 1255 dmg fore at least)

-If in the meantime the sun comes up then it looks like it gets paused until the next night. This is where i'am right now. Gl.

Tips: i did not built a trap for the summoning area but will be helpful. If you want to go straightforward without preparing an area then i recommend fire wyvern. And thylacoleo in case of alpha deathworm.