Can you sacrifice any of your character's at any time?  There is a deep one and ancient one combo for the Cthulhu deck that is a nasty combination, it seems to allow you to destroy three of your opponents characters every turn.  You sacrifice the deep one, then pay two to put him in play.  His text let's you detroy an opponents character.  Seems almost unstoppable.

frogman29 said:

If you're talking about Deep One Rising, it only works when you play it from hand. Using Hydra to bounce it back into play from discard doesn't trigger its ability. And no, you can't just say, I'll sacrifice this character, you need something that allows you to sac them.

 For the Deep One Rising ability, why does the ability not activate when it enters play from the discard pile?  Is the discard pile considered in play then?

 Well, "Play" is different than "Put into play."    Hydra puts the Deep One into play, but when you take it from your hand to the table you "play it"

The FAQ addresses this specifically in the section entitled "Put into Play" vs "Play" vs "Enters Play."

Here is an excerpt:

To “Play” a card is to pay all costs and follow all play restrictions in order to bring the card into play from a player’s hand. Card effects that take place when a card is played do not work if the card entered play through any alternate way.