In this I'm going to explain a Demoster's feats. I'm mainly going by the most powerful Demoster (Joshua Flint) but since he's a hybrid I'm also going to do the Grand Demoster as well.
First off Joshua has been seen to survive multiple black holes crashing in on him. The Grand Demoster was shown to be able to create multiple new multiverses and destroying billions of others in the process. He recreated them all again in a few nanoseconds. Joshua was able to punch a hole in the Milky Way with ease and took down a Celestialsapien just by touching it. Demoster's have absolute biology allowing them to be the peak, supreme species in the multiverse. The Grand Demoster was able to dodge attacks faster than reality could even comprehend. A rogue Demoster was able to destroy half of reality in seconds. When Ben first used Ultraversal he used him to separate two different multiverses that were colliding with eachother.
Demoster's have a strong enough telekinesis to move dimensions. Joshua was able to revive himself multiple times over and over again with relative ease. Ben used Ultraversal to fight multiple Naijins and Celestialsapiens at once and won. The Grand Demoster was shown to unable to be wiped out of existence and when he was he came back a few seconds later and rewrote reality after it was drastically altered.
This is why Ultraversal is probably the most powerful transformation Ben has.
To go along with this chunk of text here is a slight redraw for Ultraversal: