That post is light on details, but it does confirm two useful things: Corpse Lily is part of the DLC's item pool, and the reveal is tied to a live Steam store page for Risk of Rain 2: Hallowed Concepts. Steam lists Hallowed Concepts as downloadable content for Risk of Rain 2, with Gearbox Software as developer and 2K Games as publisher.
The important qualifier is that this is not a release announcement. The X post tells players to wishlist the expansion, and the Steam page says the content is "not yet available" with a planned release date of "To be announced." In other words, Gearbox and 2K are in the "come look at this thing we are making" phase, not the "you can play it now" phase.
The Steam page does offer a little more context on what Hallowed Concepts is supposed to include. Its description mentions new stages and "two new Survivors," alongside curses, enemies, and trials, all wrapped in a darker, fungus-heavy theme. If you're the kind of Risk of Rain 2 player who judges an expansion by how badly it plans to ruin your build in new and interesting ways, that is at least a promising start.
For now, though, Corpse Lily is mostly a name and a tease. The confirmed part is simple: it is one of the new Hallowed items, the DLC has an official Steam page, and that DLC still does not have a release date. Everything else will have to wait until Gearbox shares more than a wishlist prompt.
