Three different adds will spawn on incoming: a diaku juggernaut, a greater avazek and Decorin Deathbringer. Decorin is the only named you will face to complete the encounter. When Decorin dies Tallon leaves.
Each add you will face has one AOE that if stacked with another adds aoe will cause large focus damage, hence kill adds quickly and do not have more than 1 add in your raid at any given time. Kill them quickly. AOE's from Decorin and a add do not have extra damage.
There will be lots of curing in this fight so 2-3 inquisitors will be helpful for you guild. Decorin has 2 aoe's to watch out for which means if you have more than one add up inside your raid you will be getting hit with 3-5 aoe's. This is an add management fight as Decorin has very little hp and his attacks are not that powerful. Only when there are multiple adds in your raid will you begin to have issues.
Focus your raids dps on the Diaku Juggernaut as this add has the most fatal aoe stack effect if his Earth Tremor aoe stacks with a sparkfiend's Stagger aoe. When your raid gets a sparkfiend add this add will constantly cast Enthralling Flames which will stun everyone around it. This add also seems to cast Stagger as the only time Stagger seems to hit is when a sparkfiend is up along with a diaku juggernaut. This add must be killed before any other add is killed or will spell trouble for most raids.
The greater avazek does very little overall damage with its magic based aoes. If your raid should pull one of the tallonite kageshins (archers) just burn it down as it has very little hp.
There will be three curses that hit your raid and one incurable effect to keep an eye out for. The only curse to cure will be Impending Doom, as this is a fail effect if it is not cured or the person with it dies. The other curses are just a force target and a small debuff. The one incurable is a random noxious effect that will dmg you a percentage of your health every tick for about 7 seconds. Do not use any abilities (except for healers) if you have this on you otherwise you will do more damage to yourself than just letting it tick. Healers should be able to compensate its effects despite it lessening your heals by 10%.
Manage the adds well. If you do not have enough grp cures in your raid you will struggle with this mob. Two healers is ideal for raids just starting this encounter. Raids with more inquisitors (two grp heal abilities) will have an advantage but it is not necessary to win.
Good Luck!
I did this HM fight about a week ago and I didn't notice any difference from the EM, at least nothing major. I forgot to parse the fight so there may have been an additional AoE or something.
whats diff on hardmode?
Any updated info on the HM version
woot, anotheer kill down./ 
Just FYI the updated strategy is a bit incorrect about the curses. 2/3 of them must be cured or you will have serious consequences. One causes entire raid to be unable to take any action (goes through stifle etc immunity) for about 7 seconds. This isnt necessarily a raid wipe but it can push a struggling raid over the edge into a wipe. The second one is a curse that only hits decorins target but causes them to be unable to target anything (this is a problem since its on the MT). It also ticks for large amounts of dmg but given theres a 12 sec grace for the first tick unless your healers are asleep its normally not a problem. 3rd curse doesnt matter as it is just the target lock from the adds.