Touch of Death

Man Ray: Touch of Death instantly kills a regular mob when it has less health than the monk?
Patrick: Yup.
Man Ray: And stronger mobs only take 35% of my health in damage instead of instant death.
Patrick: Yup.
Man Ray: So if I lower the the mob's health to have less health than me, I should be able to kill it instantly.
Patrick: That makes sense to me.
Me: *Uses it on a mob with less health than me*
Touch of Death: *deals 35% of my health instead*

To clarify how rank 2 of this ability works, it can only be used on an enemy when their hp is at 15% or less, at which point the icon will light up.

However, if the enemy's current hp is higher than your current hp, it will only deal 35% of your max hp in damage.

In other words, if the enemy's hp is between your current hp and 15% of their max hp, you won't 1 shot them.

In order to deal full damage and instantly kill them, you will have to wait for the enemy to have less hp than you. Dont mash the button as soon as it lights up.

Rank 1 (level 10):

You exploit the enemy target's weakest point, instantly killing them.

Only usable on creatures that have less health than you.

Rank 2 (level 44):

Touch of Death can now be used on targets with less than 15% health remaining, dealing 35% of your maximum in damage

Clarification:

Shadowlands Content Update Notes posted 23 November 2020 says...

Touch of Death – You exploit the enemy target's weakest point, instantly killing creatures if they have less health than you. Deals damage equal to 35% of your maximum health against players and stronger creatures under 15% health and has a 3 minute cooldown.

Note:

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• Rank 1 is "creatures"
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• Rank 2 is "players" and "stronger creatures" (in-game uses "targets")
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I'm not really sure what is meant by "stronger creatures" because at levels 12-16 I have been able to one-shot several Rare creatures at 100% health and instantly kill Dungeon Bosses with less health than myself