This is the first article on this news, so I couldn't find a non-paywalled source. For those who don't subscribe to WSJ, the tl;dr is as follows:

• SF-based hedge fund, Partner Fund Management LP, is suing "Theranos and its founder", alleging they lied to attract nearly $100M in funding.

• Sources are an internal "fund document" and "people familiar with the matter."

• The fund document was reviewed by WSJ and states:

"Through a series of lies, material misstatements, and omissions, the defendants engaged in securities fraud and other violations by fraudulently inducing PFM to invest and maintain its investment in the company"

• Suit was filed in Delaware Court of Chancery this afternoon.

• A Theranos spokesperson responded:

"The suit is without merit and Theranos will fight it vigorously. The company is very appreciative of its strong investor base that understands and continues to support the company¹s mission."

The rest of the article is catch-up background on Theranos' controversy.