Your main tank in a raid is dying. What could the reasons be for this?
1) Your tank is undergeared for the encounter.
Go back and get more gear. In a 10 man encounter I personally think that the tank should have enough health to survive three melee swings in a row. This means that where the boss swings for 12k, you will preferably have 37k health. This means that even if you get a relatively unlucky avoidance streak your healers should have about 6 seconds to get a heal in before you die.
In general, it's a good idea to consider your tank's buffed health as a multiplier of boss melee damage. If the boss hits for 15k, for example and your tank has 29k health, then two consecutive unavoided (and unblocked) melee strikes will kill the tank. This will happen very often, and so you need very intense healing on the tank to keep him alive.
2) Something in your strategy is wrong
Perhaps your healing assignments are off, or you are having range/LOS issues. Consider carefully whether or not there is something you could change in your strategy to make the raid's job easier. It may be that you need to change your healers round, or that for certain periods you need a second healer to help healing the tank.
3) Your tanks are messing up
Are they tanking Steelbreaker so that he's standing in the blue runes? Is Razorscale being tanked in the blue fire? Anything blue, runic or on fire is generally bad for tanks or bosses to stand in.
Is the boss appropriately debuffed with an attack speed debuff and an attack power debuff?
4) Your healers are messing up
Hope it isn't this one. Healing is a bit of a dark art, and healers tend to be quite touchy - probably because it's very easy to blame the healers for a wipe without looking at what everyone else is doing. Trying to tell a healer how to do their job is a good way to start an argument. There is lots of stuff healers should be doing - starting the heal before the boss swing hits, for example, but often it's hard to tell whether they are doing this or not. Sometimes it may be possible to give your healers a kick and they'll concentrate more and improve. Sometimes it may not.
The curse of continuity
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