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For the ritual spell, you need the ritual gemstone from the first temple, then you must find a secret room in the second temple, AFTER you have finished it and unlocked the fish transformation. On the third floor of the temple, there is a lever to flood/unflood a corridor. Flood it and use the fish transformation to swim through it. There should be a new passage open, and that's where the secret room is.

For the pocket dimension, use detect magic in your bedroom in Tarkas. The mirror should shine. Interact with it to find a magic room. Use push on the button while being as close to the door as possible, so you can run inside before the door closes again, to find the book to learn the spell.