Scourgebane

An overview on which fights this enchant (140 AP against Undead) is a good upgrade from Massacre (110 AP flat), from a hunters PoV.

Lord Marrowgar: both boss and bonespikes are classified as undead. Clear dps increase.

Lady Deathwhisper: the boss is undead, and my guess is that it also affects her mana shield. Haven't confirmed this though. Most of her adds are humanoids, except from the reanimated ones. On normal mode, if you have two weapons available, I recommend switching after her shield is down.

Gunship Battle: all humanoids, bad enchant.

Deathbringer Saurfang: boss is undead, but the Blood Beasts are classified as beasts. Overall I feel this provides an upgrade on damage done.

Festergut: boss is undead, clear dps increase.

Rotface: the Oozes are uncategorized, but they're not dps-ed anyway. The boss is undead, this enchant provides a clear dps increase.

Professor Putricide: the Volatile Oozes and Gas Clouds are uncategorized, but the boss is undead. If your raid has trouble killing them fast enough, you'd want to use Massacre. Otherwise, Scourgebane provides an overall dps increase, especially in phase 3, with the soft enrage.

Blood Prince Council: the Kinetic Bomb is uncategorized, so if you're the ball juggler, don't use this enchant. Otherwise, it's a dps increase, as all bosses are undead.

Blood-Queen Lana'thel: boss is undead, there are no adds, clear dps increase.

Valithria Dreamwalker: the boss is a dragonkin, but as you don't dps her, this doesn't matter. All the adds are undead, so this enchant provides a clear dps increase.

Sindragosa: the boss is undead, but the frost tombs are uncategorized. My (10-man) logs show that less then 20% of all damage done in this fight is against frost tombs, the rest is on the boss. These numbers, together with the fact that in my opinion hunters aren't the most effective dps against frost tombs, makes me think that this enchant is a decent dps increase on this fight.

The Lich King: every creature in this fight except Ice Spheres is undead. A clear dps increase.

The glow effect is the same as on Lesser Elemental Slayer, i.e. faint red.

More importantly, most mobs in Naxx are undead. This enchant should be very popular for entry-level raiders, though possibly only if they have an alternative weapon for PVP.