From Polygon: House of X and Powers of X are dense books. On top of all the X-Men lore references in House of X and Powers of X, the twin miniseries contain tons of exposition in the form of very graphic designed dispatches from... somewhere. Some of those dispatches contain a new mutant alphabet, introduced in House of X #1, and at the end of every issue words in that alphabet give a little tease for the next one.
For House of X #2, that tease read “The Curious Case of Moira X,” presumably referring to Moira MacTaggert, the Scottish geneticist and old flame of Professor X, who’s been a vital part of the X-Men mythos since 1975. Introduced in the third issue of X-Men that Chris Claremont ever wrote, she’s been one of the most stalwart human allies the mutant super team has ever had, working up until the moment of her death to deliver the cure for the Legacy Virus.