In this question, it was decided a goblin could be permanently True Polymorphed into a lump of adamantine. True Polymorph can also turn a goblin into a larger creature, like a cow. This effectively increases the mass of the goblin.
If a wizard were to turn said goblin (poor, poor Squee) into a lump of adamantine, can the wizard specify how large a lump he gets? Are there any limitations on how large it could be? For example, can the goblin become:
- A large boulder-sized hunk of adamantine
- A dragon-sized hunk of adamantine
- A mountain-sized hunk of adamantine
Best Answer
The answer to this is in the description of the spell itself.
PHB. pg. 284 (continued from 283 True Polymorph spell description); emphasis added on the applicable point is mine
Object into Creature.
This stands to reason that this limitation is applied both ways. Although the Creature to Object section doesn't explicitly cover it, since Object into Creature does cover the size limitation literally right before it, it stands to reason that changing a creature into an object limits the size of the object to the equivalent mass of a creature within the CR 9 rating limitation. So if you wanted to turn a chipmunk into a house, it wouldn't fly. In addition, this prevents the abuse of turning a mountain into a mouse, and then letting the spell wear off inside a town you don't like anymore.