Radha's Firebrand

Whenever this creature attacks, target creature defending player controls with power less than this creature's power can't block this turn.
Domain — {5}{R}: This creature gets +2/+2 until end of turn. This ability costs {1} less to activate for each basic land type among lands you control. Activate only once each turn.

Magic Online Promos

Dominaria United Promos

Dominaria United

Whenever Radha's Firebrand attacks, target creature defending player controls with power less than Radha's Firebrand's power can't block this turn.
Domain — {5}{R}: Radha's Firebrand gets +2/+2 until end of turn. This ability costs {1} less to activate for each basic land type among lands you control. Activate only once each turn.

Domain abilities count the number of basic land types among lands you control, not how many lands you control or how many of any type.

Increasing the target creature's power in response to Radha's Firebrand's first ability can cause it to be an illegal target for the ability. However, once the ability has resolved, it won't be able to block even if its power becomes greater than Radha's Firebrand's power.

The basic land types are Plains, Island, Swamp, Mountain, and Forest. Land types other than basic land types (such as Desert) don't contribute to domain abilities.