This could be really overpowered with linking to carries : )

I could decrease the attackspeed even more, to something like 2.0. Since I add pure damage and not dps it would make the ability weaker. (Since the 20% of the carries dps would be dealt one time each two seconds. == 5-10% of the carries dps as damage.)

I don't think it would be extremly imba, since it's like a worse version of a 10-20%/5-10% increased damage taken debuff. (Only on one target and the damage is further reduced by armor and resistances.)

It just seems like it is overly strong but:

Forgotten Soul Increases spell damage taken by 86%
Skeletal Sorceror Increases damage taken by 35%
Dark Storage Increases damage taken by 28% and spell damage taken by 14%. And most times for the whole wave.