The Momento is an item that, much like the Totem of Undying, will activate upon death. Unlike the Totem of Undying however, it will not prevent death. The Momento will keep the player's current inventory and experience upon them respawning after dying. This will consume the Momento and further ones will need to be crafted.
The Moment belongs to the Calliope collection and does have a prerequisite to unlock it's recipe. This prerequisite demands the player that 2700 undead mobs be slain. The Momento does have a very expensive cost to craft requiring:
The Momento does not need to be held in the player's hand or offhand for it to activate. The momento just requires that it is in a player's inventory upon death.
The Momento upon death does keep the player's experience points.
If both the the Totem of Undying and Momento are in the player's inventory upon dying. The Totem will be used over the Momento since the player hasn't technically ‘died’ yet.
The Momento can be stacked allowing for multiple deaths to occur without the player losing their inventory.
Due to the Momento keeping the player's inventory upon death, upon returning to where the player died a grave will not be present.
The Momento is arguably better or worse than a Totem of Undying depending on the distance from the player's current respawn point and what ever the player may be doing at their location of death.
A possible advantage of dying proper with the Momento over the Totem of Undying is that the player my be able to swap out their equipment or gather materials needed to repair their current gear. This however is quite situational.
The Momento, if allowed, makes for a great item to have during PVP games as it allows the player to get back to fighting without the need to pick up items from their grave again.
This by extension also prevents players from camping another player's grave.