With Delves update you can notice a new category in your Collections under Combat – Mementos. There are 3 sub-categories in there – Creature Mementos, Boss Mementos and Biome Mementos.

You can find Mementos in chests that spawn after completing every Delve floor. All mementos are untradable and expire if not used within 3 hours after picking them up. However they can be loot collected.

Note: Challenge Delves floors rotate weekly, and so within one week one floor (depth) will feature one biome. For example, if on monday you see Confectious Caverns in Challenge Delve on depth 120, till next monday reset it will always spawn Confectious Caverns on depth 120 for all players. For mementos this means that if one player got a memento in Challenge Delve, all other players can get that same memento from that same depth this week.

Update: With the Deep Dragons update Creature and Boss mementos can be deconstructed into Memento Shards which are used to craft portals to specific delve Bosses at Delve Workbench. Read more at https://trovesaurus.com/deepdragons/gateways .

Creature Mementos:

• 74 different mementos
• can be found in Delves that feature those monsters
  • Example: Memento: Hive Bee can be found in The Hive Delve
• worth 5 Trove Mastery Points each
• deconstruct into 1 Uncommon Condensed Memory

▼Creature Mementos

Boss Mementos:

• 31 different mementos
• can be found in Delves that feature those bosses
  • Example: Memento: Triceratops can be found in Primordial Pit Delve (Jurassic Jungle)
• worth 15 Trove Mastery Points each
• deconstruct into 1 Rare Condensed Memory

▼Boss Mementos

Biome Mementos:

• 11 different mementos
• can be found in Delves of appropriate theme
• Example: Memento: Subnautic Passage can be found in Subnautic Passage Delve
• worth 25 Trove Mastery Points each
• deconstruct into 5 Pressure-Locked Delver's Caches and 100 Inert Geodes

▼Biome Mementos

You can use a very handy Google-document for tracking Mementos thanks to awesome Trove community members [ LunarStation ] and [ Criteox ].

Click on the File > Create copy to use it for yourself.

https://docs.google.com/spreadsheets/d/1ymlTA_4QCFFiM7im2lOlgPT6ISKisSsoXCa0SZrbvoQ

The list is updated regularly by various trusted players who keep track of weekly rotation of Mementos, Bosses and Merchants that can be found in Challenge Delves.