https://imgur.com/a/0ZUkZUC

V) Opens passage at V1.

1) Underwater platform – hold your breath to spawn a magic bridge/teleport.
2) Ornate key dropped by Dolmen at 2a), used to unlock door at 2b). Lever SSW of 2b) opens passage to the Dunes. A key lock in a wall just SE of 2b) opens a teleport to the southern isle (accessed by crossing the river near X).
3) Use rusty lever multiple times to lower the bridge.
4) Tunnel through the Grotto leading from a) to b). Below, find the timed backdoor teleporter
that appears after opening the impassable door.

5) Visit the circles (underwater platforms), then return to step on the plate.
The "X" will no longer teleport projectiles and allow crossing the river.

6) Levers at 6a), 6b) and the pressure plate at 6c) open the entrance at b).
7) Use the dark bolt spell or dark wand to "bring the dark".
8) Pressure plate opening the ruins at 9a).
9) Defeat the crab at 9a) to open the sealed ruins at 9b). Use another ornate key to open the exit to the west and spawn a bridge over the river to the east.