He can create portals to either teleport something somewhere else, or suck them into oblivion. He can also shoot energy beams from portals he opens up.
The Black Guardian of Ulyaoth has several jellyfish-like features, with three legs, and likes to stomp on his victims.
If the player has Pious Augustus choose Ulyaoth as the Ancient he wants to summon, Pious can be seen talking to him several times. In the end, Alex summoned Xel'lotath to fight Ulyaoth. Xel'lotath damaged him with an energy ball and an electrical shock, before reflecting several of his energy beams back at him, and then she powered up and struck Ulyaoth with green lightning, killing him.
If the player has Pious choose Chattur'gha as his Ancient, Alex will summon Ulyaoth to fight him. Ulyaoth destroys Chattur'gha by sucking him into disintegration portals, but it is soon revealed to Alex that Ulyaoth will inflict the same damage that Chattur'gha could have inflicted, and so she and Edward's ghost cast a 9-point Bind Spell to seal him away. However, Mantorok somehow tampered with the three timelines and allowed them to continue as one after the deathblow of each Ancient, leaving Ulyaoth destroyed forever.
If the player has Pious choose Xel'lotath as his Ancient, Ulyaoth is briefly shown in a vision by Xel'lotath, being beaten by her, though he is not shown clearly enough.