The Gatewarden at Candlekeep approaches you as you pass through the south-eastern section of the fortress and offers a session of training as the leader of an impromptu party in the Storage Cellars. This helps you understand how to fight in the game.

[...] I'll not offer it to ye twice.

— Gatewarden, as he offers it for a second time

"Soon be gone, will ye? Aye, Gorion arsked me to teach you a whit about the finer arts o' mass combat 'afore ye leave. Can I pique yer int'rest with it, [you]?"

1:-Yes, Gatewarden. I would appreciate that.
Good, then, child. 'Tis one thing to maintain control o' yerself in the heat of battle but 'tis quite another to maintain one's authority o'er a group. I've arsked Obe the illusionist to run through a few simulations fer ye down in the storage cellars. Just follow me this way an' I'll unlock the door fer ye...
2:-Gorion's in a hurry, Gatewarden, and I'm afraid I can't spare the time. Sorry...
(He won't take no for an answer quite so easily as that.)
He offers again:

"Are ye sure, little one? It would be best ye took the time to learn it now rather than curse yer ignorance later. Gorion himself arsked me to do this deed and I'll not offer it to ye twice."

1:-Very well, then. Teach me what you can for I have much to learn.
(same as 1:- above)
2:-Whatever little knowledge I have, I am sure it will be sufficient, thank you. Besides, Gorion will protect me should we encounter any trouble.
Aye, ye be as ye've always been, then. Worse than tryin' to talk sense to Ulraunt, himself. Now there be duties that need attendin' to and I'll not suffer a brat like ye any longer.

If you refuse his offer both times he walks away, (apparently into the Clinic, but you will not find him there if you follow), and does not return. The Storage Cellars then remain inaccessible; not so much as a locked door is to be found.

If you accept his offer and follow him:

"There ye be, [you]. If you haven't trained with Obe yet, then the door's wide open and all ye needs to do is drag yer cursor o'ertop it. If you've already done yer training then you should go and see Gorion. Yer father is anxious to see you."

If you speak to him again later:

"Hello there! I thought I should tell you that Gorion has been looking to talk to [you] and all of his friends. You should go talk to Gorion as soon as possible."

When the player returns to Candlekeep in Chapter 6, the Gatewarden will be on the 6th floor of the Candlekeep Library and will arrest the player for killing the Iron Throne leaders (whether you actually did or not).