Hi there!

New Warhammer Invasion player here, so will probably be asking a few questions as I go along learning!

In the one of our games Player 1 played Slaanesh's Domination. In Player 2's hand, Judgement of Verena is revealed.

Can Player 1 play a development and then select to play the Judgement of Verena, or does Slannesh's Domination's effect "end" if he plays a development?

Thanks in advance

Hell no.

You can only play Actions in response to cards being played, and dropping a development isn't an Action.

Also you can only play Judgement of Verena on your own turn, so if it's your opponents turn and you play Slaanesh, even if it (Judgement) was in your hand it's not your turn so you can forget about playing it.

When you play Slaanesh and the card effect is beeing resolved then you can't play anything until it's effect is finished. Except in this case where you can play your opponents Judgment of Verena both players would have the chance to respond with one or more actions. So you can't play a development even if it is during your Kingdom Phase. If you have played Slaanesh druing your turn, then you can use Judgment of Verena else you can't.