Based on the CYOA by Lone Observer.

Yanked from his everyday life nearing adulthood, an ordinary protagonist learns of his one in a hundred million gift of "Resonance", the ability to pilot enormous mechas known as "Slayers".

As the only weapons effective against Outsiders, trans-dimensional beings from an unknown origin, he's now forced to fight against legions of eldritch nightmares bent on eliminating not just this new world, but every world thereafter.

The likelihood of death is high, escape is not an option.

Not every story has a happy ending.

Original CYOA - https://imgur.com/r/makeyourchoice/fRsuzLv