Sigil Stone (Skyrim)

You obtain a Sigil Stone in Skyrim by levelling your conjuration to a high level and speaking with the master conjurer in the College of Winterhold (My Conjuration was maxed at 100 when I discovered this). He teaches you how to summon an unbound Dremora. You use this spell above one of the dormatories in the candle-lit circle to summon a rather angry dremora. Now you get to break him by killing him and summoning him a few times. When he finally relents, you order him to fetch a Sigil Stone. He disappeares and you summon him again. He hands your sigil stone to you, you lend the sigil stone to the master conjurer who grants you the ability to summon a Flame Thrall (a permanent flame astronach). He hands you your Sigil Stone back, saying that it's "Yours."

The Sigil Stone can then be placed at the Astronach forge. What it does, however, is a mystery to the author of this article. Known combinations such as summoning Flame Astronaches or creating Void Salt are unaffected by the sigil stone.