I'll reply in the game thread too to make sure that everyone gets the information:
Mardak can see that the room beyond the balcony has a huge, decorative dome-shaped ceiling. This room looks well-decorated but you can't see colour with darkvision. Mardak can't see any undead yet, he only knows that Grizel has announced that there are two on the chairs. They are obviously sentient and have not expressed any hostility yet.
Holy water isn't the 'Attack action', it's a separate action so 'extra attack'/dual weapon fighting can't apply to it. Also, holy water counts as an 'improvised weapon' which means that you don't add your proficiency bonus to the attack roll. Be careful not to waste that stuff. It could be gold dust in this place!
Also be aware that these huge chairs provide cover: total cover from behind, 3/4 at the back from an angle, 1/2 from the side, which means the AC to hit these things could be quite high if you don't go around the front of the chairs.