This is what people are seeing with Lethal Arrow.
After going through the video slowly I see that the player casting Lethal Arrow is doing nothing but casting Lethal Arrow in a normal way. There is no weaving (out of range anyway) no dodge roll, no block, just sitting there casting the ability. The full animation is there too, just like normal. The only thing not normal is that there is no arrow animation on my screen, and there are no damage tick numbers on my screen.
After the player casts Lethal Arrow they just disappear into the void until I explode.
This gives me the very strong impression this is just a bug and not an exploit. I have not experienced this consistently from the same player in any BG match.
https://youtu.be/tHafrKojsa8
I am on Xbox NA server, this was while I was in Battlegrounds, I have not played in Cyrodiil since update.
Gamer tag: DasPanzerKat NA Xbox One
1300+ CP
Battleground PvP'er Waffennacht' Builds