When Clyde traps Josef inside a mirrior, Josef activates a safety mechanism to teleport everyone away according to the extent of their power. Being the most powerful, Clyde gets teleported the furthest to the frozen ice floes of Torle.
Clyde is an extremely powerful mage; with much of his ability set consisting of magic spells. These range from minor spells such as Fire Arrows to Forbidden Arts such as Seal of the Abyss. He can also make use of the spell Detect Blood, which reduces your LP defense. Aside from magic, Clyde has abilities of his own, which involve the use of bats. Clyde's physical attacks have a moderate chance of causing Blindness and Confusion, though they can be blocked with Shields or parrying weapons.
Defeating Clyde involves using LP-damaging attacks that can end the battle swiftly, as dragging the battle on will only allow him to stick you with Status Ailments. Stunning attacks such as Grasshopper or Earth Dragon can also make things easier. Due to his abundance of Magic, it's best not to use too many Combos, as he will be able to easily interrupt you.
It should be noted that initiating this battle automatically inserts Josef into your party. It's best to keep Josef in the back row, or out of the battle altogether, as he is ill-equipped to defend himself against Clyde; despite the "Holy Seal" spell he begins with.
For all the lines of the character, go here:
According to the game guide, the magic tablet of the forbidden spell in the game is the same thing as that Clyde has been seeking for. In other word, the forbidden spell really exists. That doesn't match Josef's words, "that kind of spell have never existed". He suggests that every magic tablet can become the forbidden one. It may confirm his words that almost all Lv.4 magic tablets have some forbidden magic.