The grendelsaur has split from the wadjetsaur and has been pushed out of the Huggs Rainforest by its ancestor. It moved into Huggs Temperate Forest which was recently devastated by a huge volcanic eruption, leaving it very little competition and many small prey items. Because of the smaller prey, the grendelsaur has shrunk in size, reaching full growth at 1.4 metres to an extreme of 1.8 metres long. The grendelsaur has lost the webbing between its frill claws, which allows the claws to be more dexterous. It now pulls itself along the ground and up trees with its now-more-muscular claws. The hindquarters of the grendelsaur will slither along behind it. The two foremost claws have become somewhat larger than the other claws as they are used as tools and weapons more often than the others. The spikes on the tail have further separated into four individual claws which can be used as weapons when the tail is whipped around to the front of the grendelsaur. The tail claws have attached muscles which allow them to pull together or spread themselves apart. Their ears are slightly more effective than their ancestors, and they use hearing to be more aware of their surroundings.
The grendelsaur will ambush or sneak up on prey. It will pin its prey with the front two pairs of claws, then it will stab with the foremost pair of claws and slice it with the tail claws. It will use its foremost pair of claws to cut meat off of its meal, or swallow it whole. It dines on carrion when it is available. It climbs trees with its claws to rest by wrapping the tail around a branch.
Grendelsaurs are solitary animals like their ancestor, the wadjetsaur, and the males will constantly roam in search of fertile females. The males will try to impress the females by unfolding the first fin on their back, which is longer in males than in females. After the female chooses a mate, the males will leave to find other receptive females while the female digs a nest at the base of a tree and buries the eggs. The temperature of the eggs determines their sex when they hatch. Unlike its ancestor, the grendelsaur will only protect the eggs until they hatch; hatched grendelsaurs are expected to take care of themselves. The young grendelsaurs will split up to lead solitary lives where they will hunt smaller prey than the adults will take along with eating carrion.