Chair Entertainment has popped Undertow expansion Path of the Elect onto Xbox Live Marketplace.
It costs 400 Points (GBP 3.40 / EUR 4.80) and adds the Elect race with a five-level campaign of its own, doable in co-op or single-player.
Unfortunately there's no word on whether there will be any more Achievements on offer.
Undertow is a side-scrolling shooter that was released on Live Arcade last December. Our only criticism was its length, but otherwise we felt it was a fun and welcome addition to the service.
Microsoft also offered it to everyone for free as a way of apologising for the Christmas performance of Live. So most of you should have it by now.
It joins Rocketmen: Axis of Evil on Live Arcade today, which we'll be offering our thoughts on soon (here's one: don't).