Free while supplies last or until 3am AEST on Monday the 11th of May. There is also a key redemption deadline of 3am AEST on Friday the 15th of May after you've claimed the game. As usual with Humble Bundle freebies you need to subscribe to their newsletter if you already haven't to be eligible.
Ashes of the Singularity: Escalation is a sci-fi RTS with Mostly Positive reviews. There's two free DLC that are automatically added to your account when you activate the key. Steam store page for information: