In a post this morning from Yannis Mallat, CEO of Ubisoft Montreal, it was stated that sales of the Season Pass for Assassin's Creed Unity have been suspended. Anyone who has already bought the game will receive the Dead Kings DLC for free.

Am I eligible to receive the Dead Kings DLC for free?

Yes! Anyone that purchases Assassinís Creed Unity on PS4, Xbox One or PC will be eligible.
Anyone who has already bought the Season Pass or Gold Edition of Assassin's Creed Unity will be eligible for a free game from Ubisoft. Surprisingly, the games that Ubisoft is offering are actually newer triple-A titles.

Am I eligible to receive a free Ubisoft game?

If you purchased the Season Pass or Assassinís Creed Unity Gold Edition (which includes the Season Pass), you are eligible to choose one free game from among a select list of Ubisoft titles. These are:

The Crew
Far Cry 4
Watch Dogs
Assassinís Creed Black Flag
Rayman Legends
Just Dance 2015

We are currently setting up the system to process this offer. We will provide an update on how to claim your free game soon.
While I'd really like for AC Unity to be able to maintain 60 FPS on Ultra, especially with dual R9 290s, this is most definitely a good move by Ubisoft after some terrible recent game launches.

Full statement from Yannis Mallat is here: An Update on Assassin's Creed Unity by Yannis Mallat

Dead Kings DLC/Free game FAQ here: Assassin's Creed Unity Offer FAQ - Assassin's Creed Unity Updates | Ubisoft (US)