Apexis Crystal

Source
There are several ways to acquire Apexis Crystals while adventuring on the flying-mount-accessible plateau of Blade's Edge Mountains.

Guardian of the Monument
There is a quest to summon an Apexis Guardian. Bring at least 4 friends (one for each big button) and use 35 shards to start the big version of the simon game. Reach level 6 and the guardian shows up. Kill him and he can drop one Apexis Crystal. (The person who spent the shards traditionally clicks Need when rolling on the Apexis Crystal loot).

Deathwing Brood Cloak
Bring a party to the nests of Obsidia, Rivendark, Furywing, and Insidion. Spend 35 shards on each egg to summon the dragon (the egg is destroyed and respawns minutes later). Killing all four completes a set of scales (everyone in the party gets one scale per dragon). Combine the scales to create a. If you do not like the enchantment on the cloak, take it to Sky Commander Keller and he will trade you an Apexis Crystal.

Fel Crystal Prism
Bring a party to a Fel Crystal Prism and use 35 shards to summon (randomly) Galvanoth, Mo'arg Incinerator, Zarcsin, or Braxxus (elites). When killed they can drop an Apexis Crystal. (The person who spent the shards traditionally clicks Need when rolling on the Apexis Crystal loot). The Fel Crystal Prism is used up by the summoning so if you want to do several you will have to travel and clear around each prism while they "cool down". There are 4 Fel Crystal Prisms in Forge Camp: Terror and 4 more in Forge Camp: Wrath.

Bashir's Landing
By using Accelerator Modules at the nether infusion machines at Bash'ir Landing will either summon a Bash'ir's Harbinger or Bash'ir Reckoner. Like other mobs who drop Apexis Crystals, they both require a decent 4-5 man group to kill. Their spell reflect is known to reflect spells with a greater chance to score a critical hit.