If you would lose unspent mana, that mana becomes black instead. At the beginning of your first main phase, look at the top card of your library. You may reveal that card if it has three or more colored mana symbols in its mana cost. If you do, add three mana in any combination of its colors and put it into your hand. If you don't reveal it, put it into your hand.

When Omnath enters, draw a card. Landfall — Whenever a land you control enters, you gain 4 life if this is the first time this ability has resolved this turn. If it's the second time, add {R}{G}{W}{U}. If it's the third time, Omnath deals 4 damage to each opponent and each planeswalker you don't control.

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, create a 5/5 red and green Elemental creature token. Whenever Omnath or another Elemental you control dies, Omnath deals 3 damage to any target.