Stealth, Poison, and Other Useful Tools, Assassin's Creed Origins. 

It's in the progesion system you unlock it by gaining points to spend in that menu.

Hold left on dpad if using a controller

@@mrgriff9t186 I’ve tried that but no poison , only sleeping ones , although I’ve got both darts !

@@frankhuman9972 apparently you can't quick select them and can only swap them in the gear menu which is a bit dumb. Sorry about the wrong info in the last comment

@@mrgriff9t186 I’ve also tried that still nothing

@@frankhuman9972 that's what it says online, it says you have to put it in the slot beside your shield because you can only have one tool equipped at a time.