Is the “的” not always required when prefacing a noun with an adjective? I've decided to brute-force my way through a novel written in Mandarin as a means of expanding my vocabulary and bettering my understanding of Chinese grammar, and suffice to say my horizons have been broadened. I've come across the following sentence, “至少两千具不死骷髅像潮水一样涌过” and I am befuddled. Broken down I take this sentence to mean:

至少　　-　at least
两千具　-　2 thousand (dead things)
不死　　-　undead
骷髅　　-　(human) skeletons
像　　　---------------⬎
潮水　　-　a tide 　 |--------⤍　resembling
一样　　---------------⬏
涌过　　- rushed forth

Put together in English, "At least 2 thousand undead skeletons resembling a tide rushed forth." The confusion is, 不死 (undead) is an adjective modifying the noun 骷髅 (skeleton). Shouldn't a "的" be inserted between the two? I've been led to believe that "不死的骷髅" is grammatically correct while "不死骷髅" is not. Am I wrong or is the author of this sentence wrong?
不死骷髅 = the "undead", or Zombies, (a fantastical possibility)

不死的骷髅 = skeletons or skulls that are actually alive, (a natural impossibility)

So, 至少两千具不死骷髅像潮水一样涌过:-

At least 2ooo Zombies went passed like the tide. 

There is therefore no adjective modifying the noun in 不死骷髅 as it is read as a single item,like "Zombie", which if broken down would read "a dead body that has the capacity to move like a living being", in other words, 不死骷髅.