According to an article, frost runes were an item the demoer found.
No, it's a spell.
Todd fights his way past the guards, solves a few interesting puzzles, and uses Frost Rune spells like landmines to defeat a giant Ice Spider before finally confronting the thief.
No, wait, maybe it's an item.
There's also an Elven glass sword and a Frost Rune.
Wait, nevermind, it's a spell.
Todd manages to wipe it out fairly quickly by spamming his Frost Rune – a spell that acts basically as a mine...
Conclusion: We have no idea.
I remember hearing something about how the best spells are "found" in dungeons. That may imply that they took the Spell Tomes DLC concept and added it to Skyrim, where you may find scrolls or tomes that teach you spells. Eurogamer talks about the Frost Rune as though it were a consumable item, but Gamesradar claims Todd "spammed" it. Escapist also uses the plural. So my guess is he found a tome which taught him the spell, and Eurogamer was too busy drooling all over his notes to notice.