Kill 'Em All, Let Odin Sort 'Em out
Kill Em All (kea) is a small AddOn to help with the various execute/heal style tools most classes have since 4.0.1.
The (extremely complicated) setup
/kea <number> will pop up a raid style warning whenever your current target falls below the set percentage in health.
Warlock Drain Soul does 4x damage on targets below 25 per cent health. /kea 25 sets the warning to pop up whenever my target falls below that, press Drain Soul, pronto.