Jvirtue55 said:

That was my concern too, with this. There is no action window when declaring attackers, so it seems that declaring Daemon Prince as an attacker would beat the forced effect to the punch. Too bad.

Jvirtue55 said:

IMO, you should declare all attackers at once.

But I'm not sure what would designer say.

U can't blank DP with Screamers and attack with your Daemon prince, becouse U declare attackers, THEN feared effect works, in other words to use feared ability you need all attackers declared.

I'm not sure this is correct. You have to declare all of your attackers before the action window but you don't have to declare them all at exactly the same instant. Feared isn't an action, it's a keyword so it doesn't have to wait until the action window to take effect. It all boils down to exactly how you interpret the wording of the Feared ability.

If "while this unit is attacking" begins from the instant the unit is declared as an attacker (similar timing to Counterstrike) then this will work. If it begins after all attacking units have been declared then it won't.

I don't want to say one way or the other at this point, but my inner-rules-lawyer is insisting that this MIGHT be correct.

Well, since I started this, I've sent the question to FFG, so that we may have a definitive answer. Perhaps, it will also clear up a bit of the timing for these kind of events.

When I said about at least three ways of blanking the text box of daemon prince I was thinking about Screamers of Tzeentch - yes Titan, you guessed it. I have to admit I was not 100% sure if it will work, I mean if the feared will kick in just in time to select daemon prince as an attacker. It will be great to hear from FFG what is the true answer here. Thanks

feared works at the beginning of ACTION WINDOW, so after all attackers are declared. there is no way to choose screamers as attacker, then use feared, then select Daemon Prince as attacker.

 Teokrata is right. Feared can't trigger until you are already attacking. There's no way to go back and add an attacker.

Here's the official answer (with many thanks to Lukas): 

"You declare all of your attackers/defenders at once, simultaneously. After they have been declared you will trigger any Constant or Forced effects resulting from this, which would include Feared (then after those resolve an Action window opens). So you could not use the Screamers to blank the Daemon Prince and then have him attack."

 It was pretty sure. Thanks anyway

 Another question: It is possible that the daemon prince sacrifices himself or is he an illegal target? I’m not sure because of the rule text “… to put a resource token on it”. This part could only be happened, if the unit is still in play. To sacrifice him lead obvisiouly to a situation, in which the second part of the action isn’t possible.

Is it allowed to use Khorvak to blank the Daemon Prince?

Khorvak doesn't blank cards, he just allows other units who are corrupted to attack, no matter what zone they are in. If the D-Prince was corrupted Khorvak could not let the Prince attack unless there were 3 resources on the Prince. I think Khorvak has more of a "global" effect that does not overrule the Prince's "specific" effect.

 The rules simply say that  "cannot" effects overrides  "can" effects