DerpRavener

Well, Shadow Scribe is a Unicorn who has an interest in dark and chaos magic. This could be a reason for them to meet, perhaps.  Or maybe he could have won a "Date" with Celestia at a Charity Auction and they could go out to eat or something? 

Hmm.  The way I play Celestia she wouldn't personally step in just because he's been taking an interest in dark or chaos magic.  She'd leave that to his instructors or local guards unless there was a really dramatic incident.  And while the charity auction is an interesting idea, I would rather have had that happen in actual RP in some capacity prior.

So the three ideas I have are: Scribe was trying to break into the restricted section of Canterlot Archives without knowing Celestia was there, and she gets curious; Scribe was doing some sort of magic experiment while in Canterlot and it backfired horribly, causing some kind of collateral.damage which Celestia needed to clean up; or he had some question about a volatile sort of magic and had requested an audience.  Let me know if any of these appeal to you.

Well, the first idea is actually a good one to be honest. We'll roll with that one. Shadow Scribe's instructor these days is mainly himself.