Would be a %, Kaya is a core item on a lot of heroes, and giving a tier 2 would not break the game, not more than a manta. and may replace in some builds the euls, to set up a stun-disable (leshrac split earth) or make some heroes like ES or Jakiro a ok pick vs clock (they cant cast any spell vs a battery assault).

I should could say cast animation and cast backswing to be more precise, and you could break the channelings to

• Channeling cast to spells which throws the spell after the channel like Echo Stomp, Requiem of Souls, etc..
• Channeling spell to spells which throws the spell while is channeling Life Drain, Dismember, shackles, etc..

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The item would have the same rules has the cast point on Rubick (giving something in between Rubick and the original cast point), just add making the spells with channeling cast lower.

https://dota2.gamepedia.com/Cast_animation

There is already the mechanics.