Note: This isn't "really" a quest (or subquest or side quest), as it doesn't show up in your Journal, as all good quests do. It is initiated via dialog, like a quest, but lacks clearly defined goals and tangible rewards.
Here you will be given temporary control over 5 companions, giving you a full party of 6 to lead.
Obe the illusionist will create harmless monsters for you to practice fighting against. Every few turns he will interrupt and ask if you wish to continue; at this point you can choose to stop training and leave the cellars.
After a 10 turns he will teleport you back out without asking. Obe's excuse is that his illusions begin to get real if he's been conjuring for too long without a break.
The monsters spawns as as follow: Gibberling, Xvart, Skeleton, Kobold, Tasloi and then 5 tuns of Hobgoblin archers. Obe will describe each monster a little before spawning them, except the Hobgoblins for which he gives the generic information. All the spawns contain 6 monsters each. When your characters stand in proximity of Obe, he may spawn new monsters before the others are killed or finish the exam before killing the last wave – however, you can move your party away to avoid the extra spawns if you wish to kill the monsters at your own pace, especially on higher difficulties when their HP are boosted.
This exercise bears no material rewards but helps you to understand how to make multiple players work as a team when fighting monsters – all the items temporary party have in their inventory are being taken away when leaving. The illusions do not drop any items and the chests contain no items. However, you can open the two closed chests with Deder's (or Gorion's Ward's) thieving ability for extra XP in Enhanced Edition – you can remove other party members and make sure Gorion's Ward portrait stays on top if keeping Deder for maximum gains). In the original game killing each illusions nets 1 XP, but in the Enhanced Edition they give none. However, killing them still counts for "Number of kills" counter and up to 60 kills can be gained toward the Murder Incarnate achievement.
While you can not take anything away, make sure you don't leave any items you brought inside your temporary party members' inventory, as these items will be lost permanently. Consumed items brought won't be restored as well. Brought metal weapons can break here as well and damage inflicted on Gorion's Ward will stay, although it's only possible to incur damage via party's own attacks.
It is also possible to heal any wounds that may be suffered before by using Canderous's special ability or healing potion from Deder's inventory. Canderous' Bless and Protection From Evil casted on Gorion's Ward before leaving the area will also persist, and so will the effect of Oil of Speed which may be handy to steal items from the Candlekeep Inn and escaping before guards appear. You can also use other member's high lore or Mordaine's identify spell to identify items (such as Dagger +1, a possible reward from An Errand for Fuller).
The items temporary members carry are of different type than the regular one, that's why they are always lost when brought to Gorion's Ward inventory and why the weapons don't break. This also make brought stackable items such as Potion of Healing and Arrows don't stack together with the items found in temporary party inventories.
The party that you temporarily take control of consists of 5 companions: Arkanis, Canderous, Osprey, Mordaine and Deder. They all have items and spells of their own. You can only use party's memorized spells, as there's no way to rest inside. None of these characters have a biography filled in, too.
You cannot bring any of the items from these characters' inventory outside the Storage Cellars.