Global ultimate idea - OP or not ?

I usually play Karthus mid, and I thought about an ultimate (not necesary for Karthus, but just the ideea). "Mark an enemy champion for a prepared death instantly, revealing him. After 3-5 seconds, deals 50% of the target maximum health as true damage." The champion possessing this ultimate have no other direct offesive abilities, just one offensive ability which makes someone in his team a bit stronger (gives AD/AP etc.), and two defensive abilities which does not help the ultimate to be better in any way possible. What do you think ? A champion who don't have anything strong than an ultimate (which is actually extremely strong) would be OP, or not ? Let's compare the situations to both an AP mid with Zhonya and an ADC without Banshee/Guardian Angel (and obviously without Zhonya).
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