Subjective question is subjective. Without knowing what champion you are asking about, there is no answer to your question.
if i play skarner whats better
Subjective question is still subjective.
It depends on your other items, runes, and masteries as well.
Lichbane = AP
Trinity Force = AD
Trinity in general, lich bane if you go the AP tanky route(with rod of ages, guinsoos, rylais, etc...)
... one scales with ap and one scales with ad... very hard to understand.
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