Finnest (meaning Hold of Ice) was an object used as a repository of power by a Jaghut. It was used as the repository of the essence or powers of an individual. Its exact form was not specific, likely a number of objects could be transformed to suit. Tool described them as a "self-contained Omtose Phellack Warren".
In Gardens of the Moon
The powers of the Jaghut Tyrant, Raest, were imprisoned within a Finnest which was in the form of an acorn. It was kept among a variety of objects in a burial chamber within a barrow in the Gadrobi Hills. It was sought out and stolen by Adjunct Lorn whose Otataral sword kept its aura dampened, temporarily hiding it from Raest's senses. Lorn buried it in the garden of Simtal's estate in Darujhistan. There an Azath House grew from it, which came to be called the Finnest House.
There was some evidence that Finnests could transform themselves and act independently as was witnessed when Ganoes Paran had to fight Raest's Finnest. Given that Raest's Finnest was found as an acorn, it was not surprising that the transformed Finnest which did battle with Paran was tree-like.
In Memories of Ice
As High Fist Dujek Onearm's forces besieged Coral, the Pannion Seer stole the powers and soul of his adoptive mother, a K'Chain Che'Malle Matron, and stored them within a Finnest shaped like a lizard's egg. He later agreed to release the Matron's soul into the Rent at Morn to release his sister.
In Reaper's Gale
Later that Finnest was used by Silchas Ruin in his revenge against Scabandari, stabbing Kettle in the caves on the Refugium. The power of the Finnest in conjunction with the Seed of the Azath within Kettle, created an Azath House in the Refugium, imprisoning Scabandari's soul and also sealing the twelve gates of Starvald Demelain.