At the end of the "Weirdmageddon", Bill was defeated, and turned into a stone statue. Though he was technically in the Therapism, his "physical form" was a statue.

Fortunately for him, some people, that being you and your friends, found the statue in the woods, and saw it's hand out, and as a joke, and wanting a funny photo, shook it's hand, which caused Bill to be summoned, your friends freaked out and ran.

Bill (human form): haha, it's good to be back! Except...he noticed instead of being in his normal normal triangle/pyramid form, he was in a different form, a human one...oho, I'm a human~

Bill (human form): he looked at you Your the (idiotic) person that summoned me right?

You nodded

Bill (human form): well, I suppose I owe ya something ey? It was..a deal after all~! So tell me, mortal, what would your incompetent mind want?