Astrum, the Star Sigiled

Astrum, the Star Sigiled can't be countered.
Forestwalk, plainswalk, islandwalk.
Other creatures you control have forestwalk, plainswalk, and islandwalk.
Creatures you control have "{t}: Add {g}, {w}, or {u} to your mana pool."
Mana doesn't empty from your mana pool as steps and phases end.