She's called the Pale Maiden pet and, at this moment, she's a very rare drop from Vladimir Darkflame in Dragonspyre. The wiki says she also comes in the phoenix hoard crown pack.
During Halloween, she can sometimes be obtained from Halloween themed crowns packs such as the Harrowing pack or the nightmare pack.
The Lady Blackhope pet (officially known as Pale Maiden) is a potential drop by Vladimir Darkflame, the final boss in The Labyrinth dungeon in Dragonspyre. Other than that it can occasionally be found in the Crowns Shop during select Throwback Thursdays as well as in the Nightmare and Harrorwing Nightmare packs during the Halloween event in October.