Polarisia, Sea Hydra

Indestructible, Trample, Vigilance
Islandwalk, Swampwalk, Forestwalk, Plainwalk, Mountainwalk
If you control 5 or more creatures, this spell costs {10} less to cast.
On each upkeep, double the number of +1/+1 counters on this spell.
This spell enters the battlefield with 20 +1/+1 counters on it.