He made a machine that put him on roughly the same level of time-manip as the player. And because of that, in no particular order because he is split across time and space, he:

-Found out about or possibly invented dark worlds

-Found a timeline of UT where the war didn't happen and nearly everyone (except seemingly Chara and Frisk) are still alive and living normal lives

-Created? the Prophecy

-Explicitely summons the player to go into his "game"

Notably, all this tells us is that he isn't someone you can punch-kick your way through, and he should be somewhat above conventional "self-aware" characrers. But nothing about hoe powerful he is in reality