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But you said the item is supposed to help tanks without damage, taunts, knock-ups, and slows.
"... with Martyr's Gambit , they can finally have an impact while waiting for cooldowns, and make good use of the armor/magic/health they have built up during the match."
"The item itself brought a lot of tanks and supports the item they needed most: something to keep themselves in lane with health regen, cooldown on spells and shields, and giving tanks the ability to be useful as a support for their team rather than just a wall with no real battle potential."
Tanks are literal walking walls. I said this item can make use of the health and armor they've built up. As a player you should see what abilities your champion has. This item's active can only be used to take damage for another player and soften the blow of any incoming damage over a short amount of time.
Your Zed should be fine if you're smart enough to be patient and attack when the moment is presented and not accidentally misuse your ult.
Most importantly, remember that there must be teamwork between the tank and the ally that's been granted the passive. It's not really meant to be used as an offensive weapon, but more as a defensive gadget to ensure that your ally lives, not go 5 v 1.