Is the “的” not always required when prefacing a noun with an adjective?
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.