Oblivion:Ambush

Quick Walkthrough
# Talk to Traven to receive your next assignment.
# Travel to the ruins of Silorn to meet the battlemages.
# Learn of the battlemages' strategy and prepare for the ambush.
# When the necromancers leave the ruins, attack them.
# Chase Falcar and kill him for his soul gem.
# Return to Traven with the gem.

Detailed Walkthrough
Readying the Ambush
Speak to Hannibal Traven to begin the quest, then travel to the ruins of Silorn southwest of the Imperial City and just southeast of Skingrad. Meet the three Battlemages (Iver, Merete, and the leader Thalfin) outside the ruins. They were supposed to storm the ruins but can't&mdash;the door is magically sealed. Instead, they've decided to wait for the necromancers to come out and then attack. Ask the battlemages about their Abilities to figure out how best to arrange them.

Thalfin will say: '' "I have to admit, I'm better with an axe than I am with magic. I've been working on spellcasting, but I still feel better with a weapon in hand."
Merete will say: "Well, I excelled in my studies in magic, especially large-scale spells. I'm pretty accurate with them, too. More accurate than swinging a sword."
And Iver will tell you: "I can manage with a weapon or with spells, but I've really been working on healing lately. I'm getting pretty good at it."''

The optimal strategy is to place Thalfin and Iver up front, and Merete in the back.

Attack!
Once everyone is in position, speak with Thalfin. Note that if you get too close to the ruin entrance, the necromancers will spot you and ruin your element of surprise. Alternatively, you can sneak to the ruin entrance, encircle the ruins from the southern side and cut off Falcar's escape route. He won't fight you, but flee into the ruins instead. If you manage to kill him before this happens, you can simply take the soul gem and leave.

Eventually, three Necromancers - along with Falcar, the former head of the Cheydinhal Mages Guild - will exit the ruins and head towards you. Once close enough, join your allies in attack. Falcar will bolt for the ruins, and you can either chase him or aid the battlemages in dealing with the remaining necromancers.

Falcar will very likely make it back to the ruins with the Colossal Black Soul Gem you are after, so follow him inside. It's a rather long path in order to find him, and filled with necromancers and their undead friends. Make your way to Silorn Sedorseli, and exit through the northernmost exit back into Silorn. This will give you access to a button which opens the way to Silorn Buroseli, where Falcar will be found. Unlike the other necromancers in the area - who tend to run around and make you chase them - Falcar will charge straight at you and attack aggressively with his dagger. He hits surprisingly hard (and fast) so don't underestimate him. Kill him, loot the Colossal Black Soul Gem off his body and leave Silorn.

Return Triumphant
Return with the gem to Traven. If Traven mentions the Battlemages, choose the Black Soul Gem topic to complete the quest.

Notes
While difficult, the patrolling Necromancers outside can be eliminated by stealth without them sounding the alarm. However, this will not advance the quest stage. The entire scenario outside the ruin is intended to force you into dungeon diving in pursuit of Falcar. The unlocking of the entrance into the ruin is a scripted event. It can only be opened one of two ways.
:Either
::* Get noticed by one of the necromancers patrolling outside. If you are seen by either of these two they will sound the alarm. Falcar will then make a very brief appearance and the door will be unlocked.
::* Speak to Thalfin and assign each of your fellow mages a duty. Once you have given all three an assignment Falcar will come out of the ruin a few seconds later.
Make sure you either have lockpicks or a spell that opens hard level locks before beginning this quest, as you will need them inside Silorn (unless you just kill Falcar outside) or you can use the button on one of the pillars
Until the entire sequence of events involving getting into the ruin is run, Falcar is non hostile. Attacking him is assault and killing him will count as murder. See Bugs. 
It is possible to sneak up on Falcar using Invisibility or stealth and simply pickpocket the soul gem from him, triggering the next stage and thus skipping the fight entirely. You won't need to enter the ruin and the quest stage will be automatically updated.
Paralyzing or over-encumbering Falcar will also give you an opportunity to pickpocket the soul gem off him before he escapes into the ruins. However, this will probably count as an assault.
If you get suspended from the Mages Guild during the quest, Hannibal Traven will still accept the Colossal Black Soul Gem. Furthermore, he'll continue to give you tasks afterwards while remaining oblivious to the crimes you've done.

Bugs
 

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