Liseiden is an eel-like species described in The Hydrogen Sonata.
Within the novel it is stated that several things tend to happen once a civilisation has made the decision to Sublime. Strange objects called Presences appear, Scavenger species (less technologically advanced species wishing to claim the departing species' technology) arrive, and any other civilisation that has kept anything quiet with regard to the Subliming civilisation customarily reveals it to them before they depart – a setting straight of the record for advanced civilisations.
The Liseiden is a scavenger species that compete with the Ronte in negotiations with the Gzilt for preferred status as to who will be given official permission to pick over the remnants of the Gzilt civilisation once they Sublime. Both have ships if not fleets on the way into Gzilt space. The Liseiden and Ronte are both indicated as being about tech level 5 to 6, as compared to the Culture and Gzilt being about tech level 8. Later fighting suggests the Liseiden are more advanced than the Ronte, at least in ships and combat systems.
Sources: The Hydrogen Sonata