Our protagonist was set for what should have been a good life; born as a noble prodigy with a talent for Dimension Magic.

Unfortunately, they did not get a chance to live that life of adventure, wonder, and accomplishment as they died an early death.

However, a kind(?) grandpa gives our protagonist a choice for the next life; instead of having a generic life starting from zero, why not join one of his experimental reincarnations not yet approved?

However, the experiment was a lot... Slimier than expected.