Make sure you have your immigrant pass with you.
There are some hidden locations in the area that you can go to that have a sidhe soldier in them. They can randomly carry one of five "trash" items. These items are non-essential to killing Telius, however they can significantly help. The items are:
The 5 locations these mobs can be found are:
Once you have all 5 of those items, or if you are skipping them, go 3 north of the gate and east. You will be at the Rowdy Tavern. Sayto the old crippled fey "help". He will take you into a hidden room and tell you about a hidden underground territory that has a captain. When he finishes speaking sayto him "I am ready" and he will take you in. Wait in this room until all of your form also arrives (each will have to say i am ready to the cripple.
After you all arrive, someone should cast light and throw it northeast. The next part can be tricky. It is a tunnel that collapses from east to west. If you are in the room when it collapses you will be killed. To avoid this, once a light is thrown northeast, keep looking into that room. When you see all the room exits disappeared that means the tunnel has collapsed. Do NOT go in or you will be killed. Once the exits reappear, quickly move ne, e, e, e, northeast (or southeast). That will put you past the collapsing tunnel.
Once past the tunnel go into the northeast corner. Cast 'detect illusion' and feel free to spell up or do whatever is necessary. Captain Telius will be to your east in a hidden room.
Telius is non-aggro so you can go in his room. He can be dispelled via the spell, slowed, plagued, poisoned and most other spellups. At archon he has a TON of hp and will take a bit to whittle down. He will cast A LOT and can do anything from spark, to druid evokes, to bolt. Keep the tank healed, and calm when needed. Counterspell him as much as possible. It is relatively safe since he is non-aggro you can calm to wait out any ill affects on self. He will emote damage to all form members based on half of the tanks hp.
If you obtained the 5 junk items, keep them in the tank/form leaders inventory. During his emote, for each item you have, it reduces the dmg he does by 10-20% per item. This can potentially make his emotes worthless if you have all 5 items.
Be sure to have loot on as his corpse will explode. Once you kill him, you will be able to potentially loot one piece of the Xaltian Smuggler's Kit set. There are 3 possible pieces he will drop: a Xaltian hard hat, ?, and ?. After the kill you will be in a dizzy room with a one way exit up and when you go there you will be in Unseelie Castle.
Note: to obtain the other 3 possible items from the Xalt set, you trade with the hawker in the Xaltian bazaar. I believe it is 2 for 1 (might be 3 for 1). If you give him any 2 items looted from Telius from the set, he will give you one of the other 3 items not obtainable from Telius in return.