The tarndrake is a large (half again the size of a man) and highly derived waterbird, its body modified into a quadrupedal and almost reptilian structure, an impression furthered by its serrated teeth, backswept horns, sharp wing claws and patterned fins along the back and tail. It is covered in waterproof feathers, overlaying a luxuriously soft down layer that keeps the bird warm in its watery habitat. The patterning across the creature's body is unique to each individual and may serve as a means of telling one another apart. Males and females are identical.
It lives exclusively in mountainous regions, where it stakes out a territory centred upon a large alpine lake. Being a derived waterbird, the tarndrake spends much of its time swimming in search of food (fish, plants, and unsuspecting natural birds all form the bulk of its diet), and can hold its breath for a timeframe comparable to a seal. Often the only sightings of the creature are a head, fin, or wingtip flashing cryptically over the waves for a second, though sometimes it may be seen basking on the shore on sunny days, or rarely in a dive, plummeting into the water from on high to catch fish (or perhaps for the joy of it). It is a misconception that tarndrakes cannot fly - certainly they seem too big for it, and they are at their most agile underwater, but fly they do, especially dispersing youngsters seeking out an unclaimed territory. It is one of, if not the, longest lived Beast Class creature - though its lifespan is unknown, it can live at least as long as a human if not much longer. If the rumours of a two-century lifespan are true, then the oldest tarndrakes must have been present at the Manifestation itself.
Its habitat requirements, longevity, and slow breeding rate make it one of the rarest of Beast Class creatures. There are as of yet no known records of a human ever becoming bonded to a tarndrake.
(and there never will be, not for many, many years.)
Once it has secured a suitable home, the tarndrake will begin construction of its hoard, a collection of items scavenged from nearby surroundings and kept in a secluded location. It is unclear why tarndrakes exhibit hoarding behaviour, but they dedicate a significant amount of time to collecting and defending their chosen items. Each individual appears to have its own preference - typically they will gravitate to natural items such as pebbles, plant material, or bones, but those rare tarndrakes living close to human settlements have been known to make off with manmade items. Regardless of origin the creature's choice of objects is fixed, and it will only display interest in objects fitting its specific criteria. For example, a tarndrake that hoards pebbles may only take note of those of a particular size, colour, or lustre, discarding any others. The hoarded items are kept in the safest location the creature can find within its territory - underwater caverns within the lake leading to air pockets are favourites. The chamber frequently doubles as the creature's preferred sleeping area, and when not hunting for food the tarndrake will spend long hours with its hoard, keeping the items clean and (in the case of organic hoards) removing any that have decayed.
(No tarndrake has been known to hoard items that humans find valuable, but that has not stopped stories of tarndrakes sleeping on underground mountains of gold capturing the imagination of beast hunters and leading to many disappointing expeditions.)
Tarndrakes are solitary unless breeding, a rare event, but they are careful in their choice of mate and display strong pairbonds although the partners continue to keep separate territories and may go for many years without seeing one another. A tarndrake seeing a mate (either sex may initiate, despite human tales) will offer up an item from their hoard to the other - the only time they will willingly part with a possession, and if the other accepts, the only time they will accept a foreign object. They will continue to exchange "gifts" many times over, as though deliberating over their choice of mate - tarndrakes seem to never be in a hurry about anything. Once they have settled into their choice the pair will raise one or two chicks, choosing the most suitable of their territories to do so in. They will stay together for several years as the chicks grow, although the visiting individual will make frequent flights back to their home territory to check that all is well (youngsters in search of a territory of their own will not hesitate to opportunistically sneak in on an older tarndrake's lair). When the young are old enough to leave and seek out their own territories after several years, the parents go their separate ways, only to reunite many years later and carry on as though they had never been apart.