Talassii are one of the Families of Beasts, representing nightmares of Confinement. This may refer to literal captivity, or a more metaphorical restriction of freedom. Due to an Anathema placed on the whole Family centuries ago, the Talassii are widely reputed to rape their victims -- indeed, the Family name is an allusion to the historical "rape of the Sabine women." However, in reality it is merely the confinement that a Talassii represents, and many of them may insist on keeping their captives in relative comfort as a way of distancing themselves from their unsavory reputation.
The Horror of a Talassii need not look like any particular thing: there are many possible ways to lure in prey. Like the Inguma, though, even a Talassii Horror that can mostly pass for a human will still have some unnatural trait or other visual 'tell' that reveals its inhuman nature. Many Talassii can pass for members of other Families, and some do so in order to avoid the stigma of the family name. 
Their primary Birthright allows them to slowly degrade the Resistance attributes of someone with whom they are in prolonged contact. The more powerful the Beast, the more damage they can do to the victim. This effect only works on one target at at time, however, and the Best cannot choose a new target until the previous victim has fully recovered.
Alternate Birthrights for Talassii include the power to suppress the Dexterity of everyone in a giving area for a scene, or to outright negate a victim's ability to resist a single action.
Talassii, ironically, may forge strong kinship with the Lost, who are victims of much the same trauma the Captors represent. Relationships with vampires and werewolves are more fraught, as both groups may encourage the Beast to indulge in the kind of abuse that reinforces their bad reputation.
The Anathema under which Talassii labor specifically gives them an unavoidable air of menace; regardless of their true mood or intentions, others perceive that the Captor is "out to get me." This makes relations with psychics and mages difficult, but leads to tremendous sympathy from Prometheans, who compare the effect to their own Disquiet.
More than any other Family, the Talassii pursue the Erasure or the Divergence as Inheritance. They grow tired of the burden of their sullied reputation and the pressure of unfounded suspicions, and turn against the Horror to end it. Those Captors who undergo the Merger or the Retreat, on the other hand, choose to 'live down to expectations' and give up the struggle against their Horror's worst desires.
A rumor persists that, if enough Talassii undergo Incarnation, they will be able to remove the Anathema that burdens the Family. So far, however, there seems to be no evidence to support that.