Blizzard has opened the first part of the Nighthold raid on heroic difficulty for testing on the PTR.
You can test yourself against Skorpyron, Chronomatic Anomaly, Trilliax, and Spellblade Aluriel for a couple of days.
We’re opening up the Nighthold raid on PTR, with Skorpyron, Chronomatic Anomaly, Trilliax, and Spellblade Aluriel available on Heroic difficulty. (The raid is actually open now – a bug was preventing access earlier.) We’ll be leaving these bosses up for a couple of days. Please use this thread to for feedback or bug reports on any of the encounters. When providing tuning feedback, please note the nature of your raid group (e.g. Mythic guild group, PUG, etc.).
Links to parsed logs are tremendously helpful.
Source: WoW forums