It is available to wardancers only. From faq:
Warcry, when casted, will grant the player an automatic boost of 20% morale. Example, if you have 50% morale before casting, you will have 70% afterwards.
While it may be of extreme usefulness, there is also a downside to this spell: you cannot cast it again for 6 ticks, and when the effect wears off, your morale will experience a reduction of 18%, which will bring your morale to a negative if it is low enough by the time the effect wears off.
This spell, like No Fear, will not stack its effect, and cannot be cast again while its running time doesn't expire.
Also regarding morale, if you cast Warcry while at 110% morale, you will still get the extra 20%, meaning you will have a max morale of 130%