Tips on running a solo session?

[Mongrel]

A new player is joining my game. He's pretty new to DnD in general, so I'd like to give him a solo session in order to get him introduced to my world (he's played a few times before so he's already familiar with basic mechanics). The thing is, I've never run a solo session before, so I'd like some tips on running one, particularly about adjusting CRs considering it's a one person party. Furthermore, he's playing a Warlock (he wanted to be a caster, so I gave him an easy one).

Mostly, I want to give him something difficult enough to be an entertaining challenge, but make sure that I don't accidentally kill him. I'm thinking that the session will be a "prison break" type of thing where he escapes from a lab where horrible experiments were going to be done to him. He will be doing so with the help of deep cover wizard/rogue of at least 15th level (the Warlock is 7th level), but I want to downplay said agent's involvement (he's basically just going to let him out and maybe help him out a little bit at the end, rather than fighting alongside him the whole time. I try not to use DMPCs if I can help it).

So, any suggestions/tips/things to watch out for?

[Crake]

If you're introducing him to the world, it would probably be best to keep it to trivial or roleplay encounters and probably keep it short, I'd assume he's eager to meet the group he's about to begin playing with, so don't put so much emphasis on the session, and you might want to run it just before one of your actual sessions and weave it into the game as everyone else arrives rather than take him aside for a day and let him wallow in what's happened until the actual game starts.

For example, if he's a warlock escaping a prison, he's probably got something on him to seal his powers, say AMF shackles. Have these on him while he's escaping to make sure he does his best to keep out of any combat encounters with prison guards who would likely be well equipped to handle him. Then have him meet his contact, and he'll remove the shackles and give him a fresh set of clothes and teleport him away, somewhere close to the party, where they will stumble upon each other or something.

What's his player's relationship with the rest of the party, in game?

[Mongrel]

Well he actually already has met the party (IRL), he's played a session with them already. He played the character of a player who recently dropped the game, but he (unsurprisingly) wanted a character of his own, and I happily obliged.

I guess what I meant more by "get him introduced to my world" is "get him into character" (though I intend to give him a proper introduction to the world he's playing in as well) The basic course of events I was going for was something like

1) Warlock is chained up with AMF shackles, the master transumeter (one of the main villains of the campaign) meets with him, taunts him, talks about how excited he is to experiment on such a good specimen, etc.

2) Master transumeter leaves, benefactor releases warlock from his prison (and removes AMF shackles), then points warlock in the direction of his equipment and gives him directions on getting to a rendez-vous point so he can teleport him away, then leaves (to keep his cover).

3) Some sneaking and maybe minor combat while warlock retrieves equipment.

4) Main passages are too well watched, warlock maneuvers through kennels to avoid notice; possibly more combat here (warlock versus horrible experiments, probably aberrations)

5) Warlock emerges into an active testing chamber, has the option of sneaking past or blowing out the containment on some nasty aberrations, loosing them on the mage-scientests

6) Warlock makes his move for the rendez-vous point, meets up with his benefactor, who reveals that the reason he rescued him was to get some sensitive information to his superior (who is also the person the party is currently taking orders from). He gives the warlock a crystal that contains his memories and uses geas/quest to ensure his cooperation (since the warlock is Chaotic Neutral, something he knows from detect alignment castings). Then he teleports him to a place close to the party (this is where the solo session ends).

Does this seem good? Anything to change, remove, include, or improve? Anything I should be wary of with this rough course of events?

[Crake]

If the BBEG is particularly cocky, it could be a good moment to give the character some plot exposition that he can present to the rest of the party? Have a bit of back and forth between him and the player before he's released. If the warlock has stuff like invisibility and flight then it might be more prudent to keep the AMF shackles on (but perhaps partially damaged so he can use some of his other abilities? Maybe it intermittently flickers on and off?) to prevent him from just breezing through the stealth and give him a bit of a challenge.

[Mongrel]

Yeah I was hoping to get some exposition out of this session. The party doesn't know a whole lot about their true enemies yet. They have met the master transmuter before but only briefly and don't really know how important he is yet (actually he isn't the BBEG, merely one of his powerful lieutenants, but he's pretty much the BBEG for all intents and purposes within the current storyline). That was a strategy I was using in order to promote some role playing from the new player when his character is introduced to the party, as he will have information that not only the other PCs but the other players don't have. I find that when character personality is established early on it helps facilitate their development so that they become something interesting rather than "Bob the fighter #52," and it seems like players like to simply say "I tell them that" rather than do so in character for information that the other players know of but their PCs don't yet have.

As for keeping a partial AMF on the warlock to prevent him just going invisible or flying away, I'm actually not too sure I like that idea. If he takes those powers, he should be rewarded for them rather than simply having his toys removed because they could break the game (especially the first time he ever has a chance to use them). I do think that trivializing encounters could be a problem, and if he was able to breeze through the whole session it certainly would be a boring experience, but on the other side of the coin if there's a stealth part to the session and he didn't put anything into hide because he has invisibility that could make the encounter significantly more difficult and thus more frustrating. Besides, the enemies here are wizards (and paranoid secretive wizards at that), they could very easily have things to detect invisible things (or just make their listen checks), and as for flight, he's in a facility that's been built into a mountain, so that would only really be a major asset once he escapes (at which point it doesn't really matter).

Don't get me wrong, I appreciate your advice and will certainly be taking your warning into account, but I think I'm going to try to design something that isn't broken by invisibility or flight rather than simply taking those abilities away (and actually I don't even know what invocations he'll be choosing yet as we haven't finished his character, so this may not even be an issue).