You must complete the following Campaign tasks to actually unlock the Spellplague Caverns! Otherwise, the only other requirement when you enter the Queue is that your Item Level is above 3100.
Heir Apparent: 36 Abandoned Treasures, 30 Evidence of Evil
Investigate Kabal: 48 Abandoned Treasures, 120 Evidence of Evil
Investigate Nostura: 48 Abandoned Treasures, 120 Evidence of Evil
Investigate Gyrion: 48 Abandoned Treasures, 120 Evidence of Evil
Confront Kabal: 56 Abandoned Treasures, 120 Evidence of Evil
Confront Nostura: 56 Abandoned Treasures, 120 Evidence of Evil
Confront Gyrion: 56 Abandoned Treasures, 120 Evidence of Evil
Unlock Spellplague Caverns: 360 Abandoned Treasures, 120 Evidence of Evil
You can earn 25 EoE's a day by completing the daily quest (Mercantile Mission); so it'll take 5 days to complete one task. These campaign tasks will take 35 days to complete. You might be able to shave off a day or two with the weekly quests as well as farming Heroic Encounters for Evidence of Evil. Once you've completed all that, find a solid group of 3500+ IL players and make your way through.
Kabal: This fight consists of 4 phases; although most of the phases are similar by design. Keep in mind the floor will overflow with lava either in the center or the outer circle.
Phase 1: Kabal is immune to damage at first and will summon enemies, a few trash mobs, followed by larger mobs that drop fire balls. If Kabal tethers chains to you and an ally, you must destroy the smaller fire balls that drop from the larger adds. When the largest mob is killed, he'll become a large fireball! Someone will have a chain attached to them and that large fireball and you must drag it to hit Kabal so you can finally damage him. Upon his death he becomes a fireball and inhabits a new body.
Phase 2: This is basically Phase 1 all over again, just Kabal has more hitpoints and a few more adds spawn. The same mechanics apply, if players are chained together, destroy the adds that drop small fireballs and destroy those fireballs to break the chains. Lead the linked large fireball into Kabal to damage him.
Phase 3: Kabal retreats to the anvil in the center and tethers all players with chains. Large fire elementals will also spawn, you must defeat them and then destroy the small fireballs they leave in their wake to break the chains. If you don't break the chains in a timely fashion, Kabal will pull the remaining tethered players into the center and explode leaving a fireball in his wake. He'll then inhabit the last shell.
Phase 4: The final phase, and it's similar to Phases 1 and 2 except there's more, larger adds and Kabal has even more hitpoints. Beat him down and finally win the day!
Giant Nothic Stone Eye: This is basically a 1 phase fight, but the Stone Eye will utilize many attacks and you have a small area to work with. Some attacks throw you straight up in the air, while others will attack the outer area of the arena making the safe part closest to Stone Eye.
When a giant eye appears on the head of Stone Eye, you must turn around and face the opposite direction or you risk a long lasting stun. The final attack shows a pulsating ring on the ground; Stone Eye will jump to that point causing a shockwave and it can push you off the edge! So make sure you're closer to the epicenter of the attack on the side of the arena with the most real estate.
Otherwise, maintain damage and eventually large mirrors will appear around the floating arena. If Stone Eye casts its stun spell and is facing a mirror.. it will stun itself and you can dps unhindered.
Nostura: Phase 1: You can damage her right off the bat but eventually she'll disappear and these large stone portals will appear. Make sure you get between them as they shoot giant rays of light across the arena. She will return and she has her own stone eye stun spell, so make sure to turn around when the giant eye appears above her head.
Phase 2: Nostura will be immune to damage and wander around. She has this annoying spell that drains a player of their life, giving it to her. Giant tentacles will erupt from the ground with some yellow portals inbetween them. You must kill the tentacles first, then have a dps character enter a portal to the far realm and kill the blue fire orbs. You can damage her in the far realm; but it is better to kill the blue fire orbs so everyone is together. Once that is done, you'll be summoned back and Nostura can be damaged again. This Phase will repeat!
Phase 3: The blue fire orbs will appear in the arena and you must kill them quickly or Nostura will unleash an arena wide spell for massive damage. As the orbs die, yellow portals will appear, run through them and then you can finish Nostura off.
Video Credit: northside