I used to know the meaning of this word in the context of what I was reading, but I've since forgotten. I remember that it had something to do with the 60-year sexaganary cycle, but I can't recall much more than that.
The word is a part of this phrase:空亡風のマーク, and is describing the hand-shaped marking on the mast of the ship.
Any and all help in understanding this will be greatly appreciated.
In the context of what you are reading, 空亡 is simply another name of 常闇ノ皇, the final boss of Ōkami. The boss is also known as Yami (="darkness") in the English version, and is symbolized by its hand. Therefore 空亡風のマーク means "the Yami-like mark/symbol". (～風 = "-like", "-style", "-esque", etc)
常闇ノ皇 （とこやみのすめらぎ）
災厄の元凶であり、箱舟ヤマトに潜む暗黒の君主。別名「空亡（くうぼう）」とも呼ばれる。 どうやらウシワカがタカマガハラにヤマトをもたらした時から舟の奥底に潜んでいたようである。常闇ノ皇
常闇ノ皇はデザインされた当初「空亡」という名前であったことが「大神絵草子 絆 -大神設定画集-」の中で明らかにされている。 それに続く形で 「真珠庵の妖怪絵巻で最後に登場して、全ての妖怪を踏み潰すという、まさに最強の妖怪。
The both links above explain why 常闇ノ皇 was initially named 空亡, which was originally a jargon word of fortune-telling. Actually some sources explain that 空亡 is a yokai created in the 21st century. See this entry and this blog article for details.
