Given the spell's description, I'd say it transforms them into a shadow per the Monster Manual.
I also would say that it grants as many touch attacks as the caster can make during the spell's duration. With the right character, you could be talking five (maybe more!) touch attacks/round!
With haste on, a level-12 caster should be able to get off three touch attacks/round; given that these are very likely to be successful against most foes, even a high-fort-save foe is gonna suffer an average of 12 points of STR-damage per round. Two rounds of this is an autokill against most creatures.
There's no way I'd allow this spell in its current form in my campaign. I'd allow it with changes; any changes would need to balance it against, say, vampiric touch, or else bump up its level. Here's one possible change:
The spell:
Sucking Touch
Necromancy[Evil]
Level: cleric 3, shadow 3, sor/wiz3
Component:V,S
Casting Time: 1 action
Range: Touch
Target: One Creature (remove this line -- touch spells do not have targets)
Duration: 1 minute/level (make this instantaneous -- otherwise it's grotesquely powerful)
Saving Throw: Fortitude Halves (make this Fortitude Negates)
Spell Resistance: Yes
The spell gives the caster a Strength-draining touch similar to a shadow's(see MM). If the caster makes a succesful touch attack, the subject suffers 1d6+1 per two levels of the caster (maximum +6) temporary Strength ability drain. A succesful Fortitude save halves the ability damage.
If the subject's Strength is reduced to 0 or less, he dies and is transformed 1d4+1 rounds later into a shadow permenently (permanently -- typo) under the control of the caster. A caster may control up to 2HD of shadow creatures per caster level at any one time. If the caster also controls animated dead(per animate dead spell), the total HD of undead plus shadow creatures cannot exceed the 2HD per level maximum. (Change this to, "A caster may only control one shadow creature created through this spell at any time. If the caster creates an additional shadow creature through the use of this spell, it immediately attacks the caster upon its creation". This prevents the spell from being used on captives or on mice and being a wholly superior version of the spell Animate Dead).
I THINK I'd allow the spell in that form. It allows a 5th-level wizard to get a shadow under her control, impossible under core rules. However, this is balanced by the danger of using the spell when there's already an existing shadow.
Of course, if you're a player and enjoy the spell, have at it -- I don't want to ruin your fun!
Daniel