Hogwarts: Capturing Nagini at the Start

Robin transmigrates into a Western magic world and gains a golden finger: a magical beast breeding cube! As long as he defeats a magical creature, he can store it inside the cube, and by feeding and breeding it through a series of methods, he can improve its magical attributes and combat power.

Nagini’s venom is too weak and can only make wounds keep bleeding?

Garden goblins are too useless and only know how to dig holes and gnaw roots?

The merpeople of the Black Lake are ugly and can only lure people to drown through song?

The Whomping Willow’s roots are deeply embedded underground, so it’s only suitable for close combat?

The Hungarian Horntail is huge but has limited vision and isn’t good at melee?

...

When the second Dark Lord leads his forces in an assault, he is shocked to find that Hogwarts has turned into the headquarters of magical beasts.

And then he and his followers get beaten by the mutated creatures so badly they can’t even find north.