Memento causes the user to faint and in return lowers the Attack and Special Attack stat of the target by two stages each.
The user will not faint if Memento fails (such as if it has no target or is blocked by substitute), if the target is immune (such as due to being Dark-type when Memento is boosted by Prankster in Generation VII), or if it is blocked by a protection move. However, the user will faint even if Memento is unable to affect the target's stats (such as due to Clear Body or if both stats are already at -6).
If powered up by a Darkinium Z into Z-Memento, if a Pokémon is switched in to replace the user the same turn, that Pokémon has its HP fully restored (even if Z-Memento was not used successfully after gaining its Z-Power effect). Due to a glitch, this effect is not applied in version 1.0 of Sun and Moon.
It sharply lowers enemies' Attack and Sp. Atk. But it drops your HP to 1 and warps you.* It sharply lowers the Attack and Sp. Atk of enemies in the same room, but it drops your HP to 1 and warps you to somewhere else.*
In the manga
In the Pokémon Adventures manga
The user causes itself to faint, but lowers the opponent's Attack and Special Attack.