Spartan is just a codename at this point, the final name for this product hasn't been determined yet. Foley writes some features of the browser may be shown off at Microsoft's Windows 10 features event on January 21 but notes that the browser is unlikely to be functional enough for inclusion in the Windows 10 January Technical Preview, it may not show up in the test builds until some point later.
Thomas Nigro, a Microsoft Student Partner lead and developer of the modern version of VLC, mentioned on Twitter earlier this month that he heard Microsoft was building a brand-new browser. Nigro said he heard talk of this during a December episode of the LiveTile podcast.
Spartan is still going to use Microsoft's Chakra JavaScript engine and Microsoft's Trident rendering engine (not WebKit), sources say. As Neowin's Brad Sams reported back in September, the coming browser will look and feel more like Chrome and Firefox and will support extensions. Sams also reported on December 29 that Microsoft has two different versions of Trident in the works, which also seemingly supports the claim that the company has two different Trident-based browsers.
However, if my sources are right, Spartan is not IE 12. Instead, Spartan is a new, light-weight browser Microsoft is building.