Delayed Blast Mine (3.5e Invocation)

You fire your eldritch blast, dealing only half the usual damage, but leave a pulsing glowing rune on the point of impact. As a free action on your turn, you can detonate this rune for your full eldritch blast damage to the creature and to any creatures within 5 feet. The rune-bearing creature has no save against damage, while any others in the area get a Reflex save for half (DC 10 + 1/2 HD + Charisma for warlocks).

You can have up to 5 active runes at any one time; subsequent runes remove the oldest placed rune. Runes persist indefinitely otherwise.