This is sort of more how a proffesional spell idea should look. I have to admit the request was not very clear and I have no World of Warcraft knowledge as I never played it so I had to come up with a totally unique idea. I advise in future describing the heroe's role in gameplay more and how your actual game plays so the effects of spells listed can be tuned to be more close to what you are after.
Corruptive Concoction
for "Master Apothecary Faranell"
Background
The Undeal Alchemist is not only a master of chemistry beyond mortal comparison, he also knows quite a lot about the dark arts of necromancy, being undead and all. When combined, these two fields result in some of the most nasty and dangerous potions ever to be created. In this particual mixture he emprowers highly reactive, unstable and messy chemicals with necro energy to create a general all porpse area effect potion for use during combat.
Effect
Effects units in an area. All afected living units will be slowed slowed. All afected undead units will gain a speed buff in stead. (these apply independantly of who owns the units) Enemy mechanical units will get a armor reduction. (only enemy mechanical) Enemy human units will also on top of that take damage over time, of which 50% is magic and 50% physical. (Only enemy humans) Allied undead will be rehealed over time. (only allied undead) The death of any afected non-mechanical, non-summoned and non-hero unit will result in the creation of a temporary weaker undead unit which is hostile to everyone with a slight chance of being owned by the Alchemist. (causes summoned undead to be created from both living and undead units which die under the effect which mostly attack anyone nearby)
Ballence
Due to the number of effects, this spell is extreemly powerful and thus ballencing it may not be an easy thing to do. Noticable weaknesses of this spell are that it can both buff enemies as well as nerf allies and the undead created on unit death of both friends and foes can be hostile to you and hinder you. Noticable strengths is that it is highly effective against enemy organic units and in the presence of friendly undead units it can also improve their strength in the process. Watchout for not using the weaknesses resulting in it becomming overpowered (if cast on a living melee attacker it will not only damage and slow them it will reheal and buff the caster). Secret weapon nerfs incase you still have problems keeping it ballenced without altering gameplay are to make it have a mana cost penalty for repetitive casts or effecting too many units, have a duration / effect penalty for afecting a large number of units or to give it a huge cooldown with duration difference.
Use
The spell is designed to be used as a main combat skill / ultimate. You could make it into a constant combat spell in which case the numbers involved should be small and duration with cooldown difference should be small like in the case of a normal spell. It could also be used as a temporary boost spell to turn the direction of combat where its effect numbers are high but the cooldown with duration difference is also high. It could even be made into a moderatly effective primary spamable skill but that may require the use of additional nerfs like increasing mana cost to stop it being too effective.
Uniqueness
Well there are very few complex spells these days sadly, however this is certainly complex. Thus it is highly unique with just the idea of the spell. What garuntees its "wow I never have seen that before, this is cool" uniqueness is the fact that no one has tried an undead potion area spell before and especially one with this many effects. The number of effects also garuntees that it will never grow old on people and will always have a use.
