Whenever you tap a Forest for mana, add an additional .
+1
Put three +1/+1 counters on up to one target noncreature land you control. Untap it. It becomes a 0/0 Elemental creature with vigilance (Attacking doesn't cause this creature to tap.) and haste (This creature can attack, and as soon as it comes under your control.) that's still a land.
-8
You get an emblem with "Lands you control have indestructible (Effects that say "destroy" don't destroy that permanent. An indestructible creature can't be destroyed by damage.)." Search your library for any number of Forest cards, put them onto the battlefield tapped, then shuffle.