At the beginning of combat on your turn, up to one target land you control becomes an X/X Elemental creature with
trample (This creature can deal excess combat damage to the player or planeswalker it's attacking.) and
haste (This creature can attack, and as soon as it comes under your control.) until end of turn, where X is Obuun's power. It's still a land.
Landfall — Whenever a land enters the battlefield under your control, put a +1/+1 counter on target creature.