Hello,
I was in the middle of my gaming session (Destiny 2) and had been playing for over and hour and all of a sudden my screen blacked out. I turned off the power and tried to reboot, but there was no image displayed. My card is put on an Alphacool water block. I had been using the GPU for over 2 months without issue (block applied ~November 5). Anyone have any ideas about what  happened? I applied for RMA and am waiting. What is the failure rate of these cards? I did have to modify the retention bracket to fit my case (because the water block is too big), will that void my warranty or affect it? Also, when I applied the water block, some of the original paste/thermal pad were discarded. How will that affect my RMA?

If you modified the bracket they can void you warranty based on that. The card must be returned in factory condition when RMA‘ing it. You will also need to replace the thermal pads you destroyed and reapply both the thermal paste on the GPU and the thermal putty on the VRM.
 
They may work with you on a fee for the thermal pads and missing putty. The modified bracket I’m not so sure on. You will need to contact them directly and see wha they say.