It would appear there are some who agree with you about it being dark. https://theweek.com/articles/838116/game-thrones-alienating-large-swaths-shows-fan-base ...All good...if it's done well. This wasn't, IMHO.
"... shot over 55 nights in Northern Ireland, it was especially important to have a perfectly calibrated screen and black-out curtains, or risk losing two-thirds of the cinematographic minutiae. Under ideal conditions, the episode was beautifully — often even breathtakingly — lit. But the episode's lighting problems also illustrated the utter disregard the showrunners have for the medium in which they're working. To understand why Game of Thrones looks so terrible on your television, you need to know a bit about how TVs create the color black ..."
OK, so I thought about this and had no real issue watching it last night. Its always dark. My wife can't stand it - its dark. We have two larger TVs, one 60" from about 2015 and one from 2017, both 4K, but this is not a 4K show. I watched this on the older, big screen because the full stereo is there and watching a battle scene with subwoofer and surround sound is so much better than tiny TV speakers. The ground shakes and its like you are at the movies. I was good with it. Its always dark.
After reading your complaint, and that article, I decided to watch the ending again on the newer TV. This is hard to explain but the older TV, although larger, is so much sharper. Thats hard to explain but its about as sharp as HDTV is going to get. But the newer one had better color and light levels. The scene at the end was a LOT better lighting. So I went back to the larger screen and rewatched the ending again. Yup, its darker and the colors not as good. The newer TV did a much better job on the lighting. Both are calibrated with some free app. So I tried to ensure the settings were identical but you can't do that. The TVs are different. The best I can say is turn off the switch that automatically adjusts the light level to the ambient light and don't let it do automatic black anything. Do not use standard settings. You need to do a custom setup with a calibration tool. There are free apps for that. But I have to say, the bigger, older TV is sharper. The newer smaller TV had better light levels.