This spirit is one that Terciel taught his daughter, Sabriel, how to summon. She suspects it is the spirit of her mother, or some residual protective magic left by her, but has no proof of this. It appears as a blurred, glowing, human shape, capable of speech, that will answer any question put to it, and protect its summoner in Death.
It is summoned by walking into Death and sending a small paper boat with one drop of the summoner's blood blotted onto it further down the river, past at least the second gate. The spirit should never be summoned more than three times every seven years, or there would be some unknown but great price to pay.
The first time Sabriel summons the spirit is when she first menstruates, and the second time is when she needs directions to Abhorsen's house.