Plane of Sky is accessed by a teleport spell. The spell is gained by doing this quest. To log the quest, talk to the giant manticore named Sektamoot who is located on a hillside next to the Sphinx Pyramid (in the NE section of the zone), north of the dwarven town you can gate to via the waysender in GM. You will also need to speak to two other manticores (located on nearby hillsides) Fentharos, and Alexandraphex.

To get the quest, you must be at least level 53.

Each of the three manticores will ask you two riddles. You must solve all 6 riddles in order to finish the quest. Often, each of the 6 quests, are major quests within themselves.

First Clue: Sektamoot

"Once man, once beast, once half-man, now cold and dead.
A servant to the unknown, spirits not of the dead
Heed commands with the ice of the earth in hand."

SOLUTION: You need to obtain the Totem of the Frozen North off of Skaldi Runehoof. He spawns in the same lair as The Unnamed King for Parathior epics, in the tomb in the ruins north of Anu village by the pond. Not a triggered spawn.

Second clue: Alexandraphex

"Shrouded in inky black on a deep blue horizon,
A thorn pierced deep in bulbous flesh.
Many arms have strained but none have set it free."

SOLUTION: You need to obtain the Harpoon of Telinan by defeating Kratas.  Killing Kratas requires getting the items for the Kratas quest to spawn him. 

Third clue: Fentharos

"Ancient armor encasing muscle and bone,
The name of a hunter engraved in the belly of a wingless dragon."

SOLUTION: You need to obtain the Ring of Garthis by killing Crocosaurus. Croc spawns in Lake Noregard and in the temple on the island (in the crocidile rock formation), and is surrounded by ancient ghariels and Tae Ew heirophants. If he's not there you can kill the Tae Ew heirophants swimming around, to spawn him.

Fourth clue: Sektamoot

"Guarded by the brood of the desert and the wind,
Nestled in a bed of sand and gold,
The fist of death engraved in stone."

SOLUTION: You must get this clue from Sektamoot by turning in one of the first 3 items. It requires killing Skahyir, who spawns in Box Canyon (map), and looting the Crown of Ragek. This Skahyir does not spawn in the same place as required for the Arios 55 (in the cave with the throne). Run North from oasis till u get to box canyon, go west around the mtn, and then follow it east all the way to a cave with two inferno drakes guarding it. Enter the cave, Ska spawns at the bottom. Skahyir is not a triggered spawn, and will require camping (Bordeal said Skahyir has an average spawn rate of 3 days. That translates into a 1% chance of spawning per hour, GL)

When you turn in the item, Sektamoot will give you a token. Skahyir is not a triggered spawn, and will require camping.

Note: You do not have to solve the first 3 clues in any particular order. Nor do you have to solve the final three clues in any order. However, you must solve at least one of the first 3 before you can move on to the final clues.

Fifth clue: Alexandraphex

"Flowing in the breath of the winter storms,
A web spun by the blue skinned cursed.
A gift to warm the cold heart of their banished mistress."

SOLUTION: Alex wants you to obtain the Queen's Shawl by killing Lady Vox in Permafrost. She is a rare dragon that spawns on the top floor of her castle. When you turn in the quest item, you receive a token. Lady Vox requires camping as she is not a triggered spawn. (Bordeal said Skahyir has an average spawn rate of 3 days. That translates into a 1% chance of spawning per hour, you can assume it's just as bleak for Lady Vox)

Sixth clue: Fentharos

"Praised as a god amidst sleeping ivory,
A coin fit for giants to guard a king,
Lies buried beneath taloned feet."

SOLUTION: This requires killing Siliskor in Elephant Graveyard.  He spawns in the canyons where there in an empty pit with a tilted statue. Siliskor drops the Shield of King Jorat.  When you turn it in, you receive a token.

Once you have all three tokens, return to Sektamoot.  He will instruct you to find a portal to the Plane of Sky on your own.  The portal is at the top of the Endless Tower (where the monks reside) in the Eternal Desert. When you reach the top of the tower, you'll get a popup and a portal will form.  Step through the portal to the Plane of Sky.  Once there, talk to the large manticore (Sarathet).  He will reward you with your permanent port spell-a tablet of Sarathet that you use to convert into the POS spell that goes into your ability menu.

Plane of Sky Map

Reward

WIZARD or DRUID:
The reward is a group portal to the Plane of Sky (which is why guilds initially placed emphasis on those classes having the first crack at the rewards - the mobs for the quest are extremely rare). You get NO xp for this quest.

ALL OTHER CLASSES:
(TY Joxxerii, Lanaz, and STRESS_FACTOR):
You don't get a scroll to memorize like a druid or a wiz; your reward will not be an ability added to your scroll book. You get a gem, called the Eye of the Sphynx, in the misc category of your inventory. In order to port to PoS, the gem must be ON you (not in the bank), and you must run out to the endless towers in the Eternal Desert each and every time you wish to goto PoS.
IF YOU DELETE THE GEM BY ACCIDENT: The Eye of the Sphynx gem is just a blue looking gem (like an aquamarine) and is easy to delete from your inventory/sell to merchant by accident. Iwright tells us that the only way to get it back is through sending in a help request. Can't be gained again via a NPC (ie. Sektamoot).