I'm pretty sure they've already said they were but just haven't said what it is yet.
Probably is. Morello hinted at it before. And I think another red post was talking about an item that drained extra mana for single target abilities to hit harder... Can't remember for sure though.
They said there is. It was leaked on EU forums, apparently it's a toggle-active that drains 2.5% of your max mana to deal that much magic damage to an enemy over 2 seconds.
Xyph already said there would be, and mentioned it's effect on the EUW forums:
Upon fully stacking manamune it changes ala AA and Seraphs Embrace
Get AD from Mana, toggle lets you drain mana to add a magical dot to auto attacks and single target spells.
How silly would that be?! An extra % of your mana on every one of your single target spells+ autoattacks you weave in? Yeouch.
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