You don't lose unspent green mana as steps and phases end. Omnath gets +1/+1 for each unspent green mana you have.

Landfall — Whenever a land you control enters, add one mana of any color.

You may look at the top card of your library any time. You may play lands from the top of your library. Coven — As long as you control three or more creatures with different powers, you may cast creature spells from the top of your library.

Trample Whenever you tap a land for mana, add one mana of any type that land produced. Whenever an opponent taps a land for mana, that land doesn't untap during its controller's next untap step.

You may play an additional land on each of your turns. Play with the top card of your library revealed. You may play lands from the top of your library.

At the beginning of your end step, untap all lands you control.

Whenever you tap a Forest for mana, add an additional {G}. +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 and haste that's still a land. −8: You get an emblem with "Lands you control have indestructible." Search your library for any number of Forest cards, put them onto the battlefield tapped, then shuffle.