Source : WoWwiki, BCWiki and own observations (up to 2nd boss so far)

In this wing of the Caverns of Time, players must defend the last Guardian Medivh, in what used to be known as the Black Morass. The Guardian, possessed by Sargeras, attempts to open the Dark Portal to Draenor, while wave upon wave of dragonkin from the Infinite Dragonflight attempt to alter time by preventing the creation of the Dark Portal.

To access this dungeon, Escape from Durnholde Keep (Aka The Old Hillsbrad) must be completed first.

Party Makeup

This instance is less forgiving than many in its desired party setup. An ideal group consists of main tank, main healer, 2 DPS (at least one should be ranged), and a hybrid capable of both offtanking and healing.

The best setup is a Warrior/Druid Tank, a Mage, a Hunter, a Rogue, and a Priest with a heavy holy build, and some in Discipline for Improved Power Word: Fortitude. A Warlock could work in place of a mage, but the warlock would have to have a very deep mana-pool (about 10k buffed)to avoid running dry on the last 5 or so portals. Not to mention a lot of Health in case things get hairy -- as the hunter and rogue would be the primary sources of DPS on the Elite that hold the portals open.

It is really important to note, that dps is key to success in this fight. Party really needs one real healer, real tank and the rest must be able to do huge dps. If killing takes too long, mana users will run out of mana. And at 2nd boss the fast killing of him is really the key (unless you have offtank and choose some other strategy).

Slow killing of wave Elites (one every time) also means portal will spawn too many small mobs and one player handling them will be unable to do it - so either they start damaging the shield of Medivh (and you have onyl FIVE beacons) or they just attack players (interrupt healing, keep hunter meleeing, etc).

Strategy

The encounter consists of 18 waves of dragonkin from the Infinite Dragonflight. At waves 6, 12, and 18, there is a boss encounter. Other waves are simply trash mobs. Once started, this encounter is timed. Additional portals will open before the current one is finished, and finishing early allows for time between waves.

Before the encounter

Before the encounter is started, clear the islands around Medivh of trash mobs. Also be sure to clear a straight path from the instance entrance to Medivh; if your party wipes on the last boss you may have time to resurrect and try again.

There are FIVE (i hope, lol) spots where portal spawns, so do not forget to really clear the perimeter around Mediv.

IMPORTANT - going to piece of ground where Medivh stands will start whole event - so make sure that noone from party goes there before you are ready to start it.

Trash Waves

Trash waves consist of one elite and non-elite mobs which will head toward Medivh and begin to attack the shield protecting him. Kill the elite to close the portal and stop the non-elites from spawning. The non-elite mobs can be handled by a mage, warlock, or hunter; other party members should focus on killing the elite.

Using a feral druid for the non-elites is the easiest method. The druid can tank the adds in bear form, thinning their numbers and keeping them from damaging the shield. A paladin or shaman also work well, being able simultaneously grab aggro and heal. Particularly in the case of using a feral druid for this role, if the mana users need extra time to drink the druid can do some healing and buy those needing mana some extra time to drink, then go after the adds again.

The elite which spawns is randomly chosen from several possible types:

• Warlock - casts AOE Shadowbolt, Fear, and a curse increasing spell damage taken by 50%
• Mage - casts Pyroblast, Blastwave, and Frostbolt
• Warrior 1 - Sunder Armor and Thunderclap
• Warrior 2 - Mortal Strike

Wave 6: Chrono Lord Deja

Chrono Lord Deja is still an easy fight. The only ability of note is an arcane blast or chain lightning which will hit for 1000 - 2000 damage. This is easily healed through, and the party member assigned to deal with the trash headed for Medivh should still be able to do so during this encounter.

After Chrono Lord Deja is defeated, your group will have time to rest before the seventh wave arrives - time is long enough to drink/bandage/eat (and manage the loot too).

Wave 12: Temporus

Temporus is tricky thanks to his Mortal Wound ability, which decreases healing taken by 10% and stacks until your tank cannot be healed. Mortal Wound will sometimes be resisted, which will also clear the debuff. There are four options available for dealing with Mortal Wound.

First, you can ignore it and simply DPS the boss down. If you choose this option, be sure your DPS classes are up to par and have whoever is in charge of shield trash ignore them. (This is probably a good idea anyway.)

Second, you can use an off tank. A feral druid, paladin, or even a rogue with evasion can offtank Temporus long enough for the debuffs to wear off the main tank. If you've been using a feral druid to tank the adds, this is a good time to change tactics. Have the druid hang back until the first add approaches Medivh, drop his beacon, then go in and start tanking the elite. If the MT dies due to mortal strike, the druid should've had plenty of time to get to #2 on the hate list. When the boss is down kill whatever adds the summoned dragon left, use a second beacon if necessary, there should be plenty of health left on the shield to complete the event.

Third, your tank can attempt some fancy footwork. The debuff is applied every 6 seconds and lasts for 8 seconds (some reports say 12), but it is only applied while the tank is in melee range of Temporus. If the tank moves out of melee range for 2 (or 6) seconds as the debuff is about to be applied, they will wear off, allowing the tank to be healed once again.

There is a fourth option, which requires some skilled mage work. The new mage ability spellsteal allows mages to steal "haste". If your tank is a paladin and they bubble they will wipe their aggro, and if the mage can keep their aggro above everyone else, Temporus will come after the mage, at which point he uses his spellsteal ability and now runs away fast enough that there is no way he can be caught. When the pally bubble is over the paladins debuff should be gone and they can re-establish aggro. if this takes too long the mage should have an iceblock ready. He is, however, immune to Slow.

Of all the waves in the encounter, this is the best one on which to ignore the shield trash and use a beacon after.

Another good option if the debuffs start stacking up too high is to position the tank on one side of the rift with a party member 20 to 25 yards from the tank directly opposite the boss on the other side of the rift, once the timer on the debuffs starts getting low have the warrior use the intervene ability on the party member and run ahead of the boss till the debuffs wear off. /fly

Wave 18: Aeonus

The final boss and wave of the encounter, this boss comes alone, without any trash headed for the shield. He himself will head for the shield and attack it, but it will take him a long time to break the shield if it have previously not taken a lot of damage. So you can make sure your party is fully ready before engaging him.

Aeonus's two abilities consist of a sand breath, which deals roughly 3000 damage in a frontal cone, and a time stop which will paralyze your entire party for 3 seconds, but will not wipe aggro. This should be a relatively easy fight (arguably easier than Temporus) as long as your main tank is kept topped off and can survive the damage taken during the time stop.

The Chrono Beacons

The NPC at the beginning of the instance will give each party member a Chrono-beacon. These are used to spawn a dragon and can be used if the shield trash becomes unmanageable. It is instant cast and you target it with a green circle on the ground. Do NOT use them near a boss, as they will despawn/die rather quickly. (It is not known whether this is intentional or a bug.) Instead use them on dragonkin that make it to Medivh's portal. You may also use them on the Rift Keepers/Lords, at least as of 1/14/07.

The trash during the Temporus encounter can be ignored if no one does anything to aggro them. Using a beacon after that encounter will clean them up quite quickly.

Wipe

If you wipe, just let Medivh die - his shield will go down in some 2-3 mins of attack. After that wait some 5+ mins, Medivh will respawn and event will reset.

DO NOT reset instance, as you will have to clear the path to Medivh and perimeter around Medivh again.