Bat Form, also known as Bat Flight,is an ability which Kain gained inBlood Omen: Legacy of Kain and also uses in Defiance . Kain first found Bat form in the first bat beacon located near the Pillars of Nosgoth. It allowed Kain to turn into a flock of bats and move across vast distances, between landmarks or places with bat beacon and reform at the desired location. The ability made a return in Legacy of Kain: Defiance fulfilling a similar role in allowing Kain to travel large distance between landmarks allowing travel to places such as Avernus Cathedral. Kain would also make use of the ability if he was injured (if his health ran out) - flying back to the last checkpoint.. He could not assume Bat form unless he was outside.
There is some difference between the Blood Omen and Defiance versions of the spell: In Blood Omen Kain had to identify landmarks and it enabled him quick travel to these at any time, though he could walk there if necessary; In Defiance, Kain could only use Bat Form at special 'vistas' which gave him a view of where he was heading and there was no chance of walking as they effectively divided the levels.
It is curious that Kain does not use Bat form in any of his other appearances, presumably inBlood Omen 2 he was too weak and had not yet regained this ability. However in Soul Reaver and Soul Reaver 2, he prefers to Teleport instead, this is likely for dramatic effect as he was an antagonist in these games.