You need to have completed the quest: Dragon Slayer, from this quest you will gain access to the wall, the wall leads to the volcano where you fought Elvarg, from there you can go up a rope to the island of Crandor.
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If you mean the caged lesser demon in the Wizard Tower, you can't pass that. However, you can attack with ranged or magic, or with a halberd, and you can pick up any drops with the telekinetic grab spell.--I believe they mean the lesser demons located in the Karamja volcano, north of the Deadly Red Spiders? To answer, you can only open the secret wall during the quest Dragon Slayer, before fighting Elvarg the dragon. Once opened, this passage will remain permanently open to use without requiring quest completion.
Karamja Volcano. Go to Port Sarim w/ 60gp and talk to a seaguy in blue. They will send you there. Head west once you get there and you will see a hole. Go through it, then walk through the dungeon until you get to a wall. Click on it. Now you pass through it and whola! Lessers.
If you mean the odd looking wall in Seer's Village beside the bank, then you must start the quest called "Elemental Workshop". As you go through the quest you will gain a key that allows you access to the workshop.
it is when the wizard attacked the bank. go to the bank guard and he will show you the video.
the only nonmember place i know of is south-east of varock by the mining site at the wall add me ~ jupitertree