But Ligh awakened a hidden power called the Unlimited Gacha.

Technically he had that ability from the start. And it always worked the same way. But since it draws mana from the environment, in the thin mana of outside world he could only summon weak cards like a rotting apple, but in the thick mana in the depths of the Abyss is where his ability truly shines. Now that he obtained the literal library of rare cards that summon him powerful allies, great weapons and cast unique spells, he can go and exact his revenge.

Personally I find the whole revenge thing pretty weak and his actions childish and illogical in many cases. The anime makes it even more simplified compared to manga. Good thing he is surrounded by colorful characters. The plot is actually not that one dimensional, although beastmen and elven story arcs are just a start and probably the least interesting, maybe because both races are portrayed as plain evil. Dwarven part that comes next is way more interesting, adding another mystery to the world, there is also ever going plot involving two competing groups of masters. Now I'm enjoying the oni part. So there is actually solid source material for better second season.