Hogwarts: Nagini captured at the start

After being transported to a Western world of magic, Robin gains an extraordinary cheat: the Magical Creatures Breeding Cube! Any magical creature he defeats can be collected into the cube, where feeding and nurturing steadily enhance its magical attributes and combat power. Nagini's venom is far too weak, doing nothing more than leaving wounds that bleed uncontrollably. Garden gnomes are utterly useless, good for nothing but digging holes and gnawing on roots. The mermaids of the Black Lake are hideous, capable of little more than luring victims into the depths with their songs. The Whomping Willow's roots run too deep into the ground, restricting it to close-range attacks alone. And the Hungarian Horntail, enormous as it is, suffers from limited vision, rendering it ineffective in close combat... Yet when the second Dark Lord leads his forces in an assault on Hogwarts, he is stunned to discover that the castle has become a stronghold of magical creatures. In the battle that follows, he and his men are beaten so soundly by the mutated creatures that they can no longer even tell which way is north...