For those of you on the PEQ server - things are a bit different for these guys.
They no longer spawn randomly at night.
They no longer wander the zone.
There is only 1 place-holder for them.
Kill any mobs on the TOP step at the Obelisk. (usually a mummy)
It has to be at night...otherwise the Obelisk is empty. (wait till about 8pm to begin)
The mob will respawn every 3 hours (that is every 3 E.Q. hours I mean)
Sometimes you'll get another mummy...sometimes it will be the Ghast.
The span of time you have to work with is narrow. You will only have a few chances to cycle through the place-holder before dawn comes...and the undead all de-spawn.
Then you start all over at around 8pm!
My tip for collecting hearts is as follows....
Bind yourself about 100 feet away from the Obelisk.
I bind myself 100 feet SOUTH of it so the big tree is on the LEFT and small one on the RIGHT.
Spend your time elsewhere till 8pm - then GATE back in and cycle things through.
Don't worry about Equestrielle or any other nasty mobs at this bind-spot. You will be safe.
Hope this helps!
CHEERS!

Cloth Cap
Cloth Cape
Cloth Choker
Cloth Cord
Cloth Gloves
Cloth Pants
Cloth Sandals
Cloth Shawl
Cloth Shirt
Cloth Sleeves
Cloth Veil
Cloth Wristband
Putrescent Heart
Raw-hide Belt
Raw-hide Boots
Raw-hide Cloak
Raw-hide Gloves
Raw-hide Gorget
Raw-hide Leggings
Raw-hide Mask
Raw-hide Shoulderpads
Raw-hide Skullcap
Raw-hide Sleeves
Raw-hide Tunic
Raw-hide Wristbands
Wooden Shield