Let's try to combine the best of both writeups...
Death Wail (Su): A shee that is hit by any successful attack will utter a horrifying scream. This attack can slay up to X living creatures within a 60-foot spread centered on the shee. A successful DC X Fortitude save negates the effect. Creatures closest to the shee are affected first. Once a shee wails, it must wait 1d4 rounds before it can do so again, and it can wail no more than three times per day. Shee do not use this power intentionally, and only wail when harmed. This is a sonic, death effect. The save DC is Charisma-based.
Looks good, how about this for the Xs.
Death Wail #1 (Su): A shee that is hit by any successful attack will utter a horrifying scream.
This attack can slay as many living creatures as the shee has racial HD, affecting a 60-foot spread centered on the shee. A successful DC
17 Fortitude save negates the effect. Creatures closest to the shee are affected first. Once a shee wails, it must wait 1d4 rounds before it can do so again, and it can wail no more than three times per day. Shee do not use this power intentionally, and only wail when harmed. This is a sonic, death effect. The save DC is Charisma-based.
That's half as many victims as a Banshee. If we want to match the Banshee's 18-killer, we could go:
Death Wail #2 (Su): A shee that is hit by any successful attack will utter a horrifying scream.
This attack can slay up to 2 living creatures for each HD of the shee and affects a 60-foot spread centered on the shee. A successful DC
17 Fortitude save negates the effect. Creatures closest to the shee are affected first. Once a shee wails, it must wait 1d4 rounds before it can do so again, and it can wail no more than three times per day. Shee do not use this power intentionally, and only wail when harmed. This is a sonic, death effect. The save DC is Charisma-based.
At the moment I'm liking #2 better.