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Navigate to Dungeon Bosses > find the dungeon > find the boss to find the ability you want a countdown for. Here we want to add a countdown to Necrotic Winds. Click the >> button

Check the countdown box to now get a countdown before the ability is cast by the boss, that's it, now you will never miss your timings!

By default, this is a 5 seconds countdown. You might want a little more or a little less, depending on the healers you play etc. You can only change this for all the countdowns, but here is how it is done. Navigate to Options > Countdown and select the duration you prefer. I like a 7 seconds countdown that gives me a little more agency, but I also play Disc and MW who have a lot of prep work.