When an event card's special ability area calls for spawning genestealers (e.g., "Choose a Space Marine. Spawn 2 Genestealers behind him."), are the genestealers drawn from one of the blip decks or do they come from the Genestealers deck?

From my reading of the rules, they come from one of the blip decks. The particular blip deck is determined by which side the Genestealers are placed to comply with the event card's directions.

However, if both blip decks are empty (last location card played), does that mean no Genestealers are spawned?

Marximus said:

From the blip pile; you read it correctly. If if that blip pile is empty, then none are spawned (and the marines will travel to a new location at the end of that phase). You won't draw from the other blip pile though. See the top of page 22.

-Tim

Tim has it right that you draw from the blip pile, but I'd add one thing. Page 14 uses exactly this event card as an example. In addition to what Tim said they specifically state that a player must choose a valid target for event card abilities if possible. Using your card as an example they state that a player must choose a marine that would spawn the most genestealers possible (two in this case). So you can't choose a marine that would result in spawning only one genestealer when the you have another eligible marine that would spawn two.

lexlloyd said:

Second that.

If you have a left blip pile of 1 and a right blip pile of 4, and you have to spawn 2 GS, then you have to spawn on a SM's right as choosing the left pile would only end up in spawning 1 GS. If both piles would be 1 thick, you could choose both, only spawning 1 (sadly enough ) and then offcourse travel at the end of the event phase (as one pile is now 0 thick).

When resolving :"Choose a Space Marine. Spawn 2 Genestealers behind him.", I always took them from the Genestealers deck. Where exactly in the rules does it mentions that they come from blip piles?

Brynjolf Irontooth said:

"Whenever a Genestealer is spawned, the current
player takes the top card of a blip pile and places it
face up in the appropriate position and side of the
formation." I found it. But I don't think that it means you would have to place them absolutely on a SM's right.

Burferking said:

I think you want to re-read the example + rule on page 14. If the right blip pile has 2+ GS and the left blip pile 0-1, you have to spawn from the right blip pile. Only case you wouldn't is if all the marines face right (and you need to spawn behind a marine).

When resolving :"Choose a Space Marine. Spawn 2 Genestealers behind him.", I always took them from the Genestealers deck. Where exactly in the rules does it mentions that they come from blip piles?

This bugged for a really long time and until yesterday, I used to spawn GS from the deck, not from the blip piles.

Then I read this topic, went through the rules and found this: p21 for spawning rules.

"whenever a genestealer is spawned, the current player takes the top card of a blip pile and places it faces up in the appropriate position and side of the formation"

Also, on p21, it says " note that some card abilities spawn genestealers. These abilities may spawn genestealers in positions that do not contain terrain cards".

Finally, p22 " Important: Spawning genestealers are always drawn from the blip pile on the side on which they are being spawned. If the blip pile on that side is empty, then the genestealer is not spawned".

All in all, this tells me that the action of the event cards "they're everywhere" and "Out of thin air" count as regular spawning action and are therefore handled like regular spawning, which to me means that spawning comes from blip piles.

Which is good, because with "they're everywhere", I use to spawn from the GS deck, which would result in a seriously difficult turn.