It's been argued a couple of times before in the "what's up with Zombies and AA's kicking so much ass" threads, and there were a few around at one point, when people were struggling to unlock the Half-Dragon - the undead status has slowly turned into a straight buff to monsters in many situations. Maybe they wanted to see if there were any where we wanted to actually have an undead to fight, like the one in Magma Mines.
The priest was also unpoplar I guess, so maybe they needed a way to incite people to try him out more, and I have to say, +100% damage against any boss sure is tempting. If it was consumable for a death protection on top of the Zombification for the next hit, and lockerable, It'd be a sweet no brainer for Priests right untill the point they nerf it
But you could actually hit the boss right away, and then go build up levels to fight him.
EDIT: And shame it's in the same pool with Soul Orb, at least untill they do something about HALPMEH and the flood of mana burning bosses. If it was a starting item consumable for a death protection and a zombification on the next hit, (with some sort of price and conversion rate, ofc), It'd be really good (possibly too good).