Huge True Damage nuke from nowhere. [GAMEPLAY]

Okay. So I was playing as {{champion:154}} Zac earlier today with roughly 6.4 - 6.6k health at the endgame, and I was preparing to use my E (Elastic Slingshot) on an enemy Jhin. Upon landing ontop of Jhin, I took a massive burst of True Damage from nowhere. I have no idea what it was until after I died almost immediately afterwards. Turns out the damage came from "game_spell_displayname_Bowmaster" and nuked me for 6270 True Damage. As there is no set ability in the game even called this, I am more than sure this is a bug of some sort. I have attached an image of the Post-Death screen showing the damage indicator and I am truly dazzled at what this is or why it happened. Some light to be shed on this would be greatly appreciated. The extra damage I took from Basic Attacks was between healing from Zac's passive.
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