Due to an error in the script, this spell actually does exactly the opposite of what the descriptions says - it replenishes the target's mana by the amount specified above instead of draining it. This can be exploited if you want to increase an enemy's mana to increase the Mana Clash damage, but otherwise the spell is useless in its current state. Unofficial fix for PC available here.
With the fix, this spell drains 2 times the mana that Mana Drain does but can only be cast half as often. The result of casting Mana Drain two times is the same mana reduction on the target but with the caster ahead by 80 + (100.0 + Spellpower) * 0.4 mana.