New assassin item ideas.

Name: think of something fancy Base stats similar to Duskblade (some AD, lethality, out of combat MS, maybe CDR). Active effect on a 90 (?) seconds cooldown: Mark an enemy champion for the next 10 (?) seconds, no channel needed, can be global. While that champion is marked, they take x% more damage from you, but you can't deal damage to any other enemy champion. Damage amplification to be adjusted for balance, a baseline 20% increase could be fine as a start. Another effect could be that your damage goes through any shields your target has OR heals AND shields on the marked target are halved, and also apply to existing shields.

What this does is that it gives assassins more power later in the game during teamfights. They sacrifice their AoE damage (Talon, Kata, Zed) for extra single-target assassination, and will also slightly help fight through an Exhaust or a big heal-shield. It shouldn't be a must-use though, since the AoE can be incredibly useful in some cases, like Kata might be able to melt the entire enemy team, or a Zed can get a good 3Q combo on a lot of members. Choosing wisely between singling out an enemy for more chances of assassination or keep the AoE aspect is crucial.

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Another idea could be something among the lines of "for the next X seconds, you are unaffected by damage reduction debuffs, also, X% of your damage pre-mitigation is dealt as true damage". X could be 20% or something.

This would help greatly when diving in only to get Exhausted or when you want to damage a tankier character, or when trying to get over enemies buying Ninja Tabi even on squishy champions.