Sleeping Monstrosity

Sleeping Monstrosity enters the battlefield with 10 sleep counters.
Remove a sleep counter whenever a player casts a spell. If no sleep counters remain, transform Sleeping Monstrosity into Awoken Monstrosity.
Prevent any damage that would be dealt to Sleeping Monstrosity and put that many sleep counters on it instead.
If Sleeping Monstrosity would be put into a graveyard from play, instead put it into its owner's library, third from the top.