This was an NPC class I fell in love with when I first read it, but never had the chance to use. In this conversion, I've taken bits from this original write up, but have based it more on the 3e prestige class, from the Player's Guide to Faerun. Seeing as how the 3e version was a master of metamagic, it fits into 5e best as a sorcerous origin.
Some of my thoughts with the build: since sorcerers don't get metamagic until 3rd level, I pushed back the cooperative metamagic ability to then, and it scales with level, unlike a lot of sorcerous origin powers. They needed something at 1st level, so I gave it the ectoplasmic spells ability. That may be of limited usefulness, but I feel it helps offset the power of later abilities.
At first I wasn't sure what mechanic to use for the stealspell and the seize concentration abilities. I finally settled on making a spell attack roll using your spell attack bonus vs. your opponent's spell save DC. As the incantatrix rises in power, it will be more likely to succeed, but it will be tougher against higher level opponents. It uses an existing number or mechanic in a different way.
After using the stealspell ability, you can scribe the stolen spell onto a scroll. Since there are not any rules for that at this time, I left the mechanics of that blank.
The Incantatrix is now available on the DM's Guild in Grognard's Grimoire: Sorcerer!