Rule 204.3.a

When Costs within Instructions appear as the first part of the effect in triggered abilities, the cost is taken to be the base cost of the triggered ability. It must be paid to finalize the triggered ability. Example: Overzealous Fan is a unit that reads “When I defend, you may kill me to move an attacking unit to its base.” In order to finalize the ability to the chain, its controller must kill Overzealous Fan.