Target land you control becomes a 4/4 Elemental creature with haste until end of turn. It's still a land. It must be blocked this turn if able.
1/22/2016 If the resulting creature attacks, the defending player must assign at least one blocker to it during the declare blockers step if that player controls any creatures that could block it.
1/22/2016 You may target a land that’s already a creature. For example, if you target a land that’s also a 0/0 creature and has three +1/+1 counters on it, the resulting land creature will be 7/7.