The Kaithim is a large lake-inhabiting creature that first and only appears in Season 1, Episode 23, The Battle of Meridian Plains.
The kaithim resembles a large predatory watery one-eyed monster that inhabits the lake of Meridian. It is actually using the energy of the lake water inside its body, making any outside attacks useless. The kaithim has three known forms. Its first form is a snake with arms. The second form is a large humanoid water fiend with a pair of arms and legs. The kaithim's third and final form is a huge towering beast that resembles a real-world hydra. The kaithim's only weakness is lightning (only will work when striking the kaithim's eye). The bolt will diffuse the Kaithim's energy, eliminating the beast.
Appearance in W.I.T.C.H.
The kaithim makes its first and only debut appearance in The Battle of Meridian Plains. It was first shown is the lake scene grabbing and eating a lone Hoogong. Later, when W.I.T.C.H. arrived at the lake, the Kaithim attacked them with its three forms. Nevertheless, the guardians defeated the kaithim in its final form with their wits (using lightning as their last idea to strike the kaitim's eye).