1) the wearer is rendered unconscious by any means other than normal or magical sleep,
2) the user pulls up the hood and invokes the words, "memento mori".
If either of these two conditions are met, the wearer will collapse to the ground, seemingly dead as per the spell description.
The cowl has two other features. Over a period of an hour, the wearer will seem to rot while actually regaining one hit point (if any had been lost). Normally, the spell would prevent the recovery of hit points, but this item is designed to protect the wearer. The illusion of rot will prevent all but the most hungry scavengers from attacking the wearer. Intelligent creatures are allowed a saving throw vs. magic to ignore the illusion and the resulting implication that something was horribly wrong with the person before being struck down.
The Cowl of Death normally has 20 charges, but charges are only used when the command word is used. Invocation of the command can be verbal or mental. Being knocked unconscious does not use a charge.
Week 1 of 52 - Magic Lamps
Week 2 of 52 - The Rat Bag
Week 3 of 52 - Emulous Cursed Sword
Week 4 of 52 - The Cloak of Peaceful Repose
Week 5 of 52 - The Cowl of Death
Week 6 of 52 - Scimitar of Smiting
Week 7 of 52 - The Symbol of Sol Invictus
Week 8 of 52 - The Equi Phalera
Week 9 of 52 - Libertatem
Week 10 of 52 - Sorrow
Week 11 of 52 - Aemilla Carna
Week 12 of 52 - The Obice Cardeam
Week 13 of 52 - The Gnollish Rattlebone
Now, the commercial. I have a little book called Zero to Hero: Uncommon Commoners, over at DrivethruRPG. I am obviously thinking of writing another and Gnolls might be the subject. Please let me know what you think in the comments.