Darkness, by the books, blocks infravision and ultravision. This is 1E PH. Nothing in the description of drow elves makes it obvious why they should see through a darkness spell. Doesn't matter if you cast it yourself- nothing in the books has precedence for being immune to your own spells.
This means that no one, not Thaka, not Threnody, no one, can see in the Darkness spell.
This means that no one *saw* Thaka go down, as mentioned in Table Talk. Your spell dropped when you died.
Being mute, Thaka cannot cry out either. You guys have one round to act, in complete darkness, with no auditory cues re: Thak's status.
If Threnody can explain how, in one round, she knows Thaka is down, and can reach into a 10' diameter sphere of inky darkness to find her silent friend, we can see how well that would work.
Imagine the situation. All of a sudden, the lights go out, Jim screams, you hear the sounds of pit ghouls gnashing their fangs. Casting Shield is perfectly reasonable. Anything you guys want to do, in the 15 seconds, plus or minus, that the lights are out- and you have no way of knowing when they'll come on- please justify if it requires acting with visual or (re: Thaka) auditory cues.
Also, if you're trying to save Thaka, please roll initiative. d10, modified by DEX.