Poisoned Weapons and Etherealness ???

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In 3 of my games, it happened that Tritons killed my GK with ease, often if the 1st round of combat, swarming him.

Once however my GK had over 70 hit points due to high dominion ... but he was killed too in the 1st round of combat.

I'm thinking a poisoned weapon ignores Etherealness just as a magic weapon. It is right?

Nope, that's just how Tritons are. If your SC relies on spells for his power, as the GK does, then tritons - or any horde of strong flying units - will be very problematic. With Tritons you will be facing 27 attacks per turn - even with etherealness you are going to be hit a lot and it only takes a couple of lucky rolls in this case to wipe you out.