Arriving from Level 7 Castle Wizard Room at K6, runes on the wall tell you 'yellow light from a yellow gem' and at E13 an outline picture on the wall shows a wizard crossing an enormous plane of nothingness.
The Earth Elemental holds power over the door at L13. We'll come back here in a minute.
At K9 you find the entrance to the Endless Void. A chasm is between you and a distant platform. A voice whispers that only golden rays with melodic tones will allow you to bridge the gap of time. The ritual must be performed correctly and you must complete it. Remembering the runes and picture (and facing south), use the Rainbow Yel (yellow gem) and a mystical golden haze floats over the void, waiting ... use the Harvest Horn and the haze forms into a solid path. A voice bellows 'It is done!'. Cross in safety to G12 and go through the door.
There is a hard fight against a gang of dragons at G15 but, through the door at E15, you enter the Earth Elemental Palace, filled with a bounty of priceless gems. It seems to shake with great power. The Earth Elemental Andreas is on a mountainous perch surrounded by Tyrans, Manticores, Basilisks and Dragons at C15. Get onto the next square (C14) as Andreas and he stops attacking you. Use Control, then slug it out, healing if you can. You win Infinite Armor, a War Hammer and Nature's Robe.
Go back to the door at L13, Andreas no longer defends it. Inside, En-Li-Kil's Phantasm Guardians stand watch over the entrance to Arnakkian's Tomb and you'll fight them at N14. Once they are defeated, moving to O14 causes a magic field to teleport you to Arnakkian's Tomb (inside the Level 7 Castle Wizard Room). The laughter of an ancient archmage fills the corridor when you arrive at I7.
Doors appear and disappear in the corners where you'll meet gangs of Basilisks and Liches. In the centre, the square at I10 is surrounded by four doors each marked with the symbol of the four corners. Work your way from corner to corner, starting from the top left (L7), thence to L13, F13 and F7. Walk north, turn towards the centre and go east up to the wall at I9 and the Wizard Door opens. Inside is the spirit of Arnakkian Slowfoot and a gaggle of Liches. A very hard fight! But you will be rewarded with a Rainbow Blue gem.
I still found there was a small area I couldn't enter along the south side of this area, from A/B7 to A/B10.