The banshee is described on page 30 of the Monster Manual II:
Wail (Su): During the night, a banshee can loose a deadly wail. This attack can slay up to eighteen living creatures within a 30-foot spread centered on the banshee, or within a 60-foot cone extending from the banshee, at the creature’s option. A successful Fortitude save (DC 26) negates the effect. Once a banshee wails, it must wait 1d4 rounds before it can do so again, and it can wail no more than three times per day.
In 3.5 edition, the rules for a death effect are as follows:
The bodak’s abyssal eyes can kill with a glance. The dreaded power word kill spell can slay without even allowing the victim a saving throw. A single arrow of slaying can fell a dragon. Even a fighter with 100 hit points can be killed by a single death attack. In most cases, a death attack allows the victim a Fortitude save to avoid the affect, but if the save fails, the character dies instantly.
- Raise dead doesn’t work on someone killed by a death attack.
- Death attacks slay instantly. A victim cannot be made stable and thereby kept alive.
- In case it matters, a dead character, no matter how she died, has –10 hit points.
- The spell death ward protects a character against these attacks
I've looked in the errata and the 3.5 update, but haven't found any mention of whether the banshee's wail is a death attack or not. It certainly fits the description. I'd be interested to hear other DM's opinions.