Yes, "tombs" can function as a plural possessive noun when indicating ownership or association with more than one tomb. Example: The tombs' designs varied in style and size.
The plural possessive form of "pharaoh" is "pharaohs'." For example, "the pharaohs' tombs were filled with treasures."
The plural is relatives. The plural possessive is relatives'.
The plural is lads. The plural possessive is lads'.
The plural form is Mexicans. The plural possessive is Mexicans'.
The plural form is suffixes. The plural possessive is suffixes'.
The plural possessive of stepsister is stepsisters'.
The plural form is echoes. The plural possessive is echoes'.
The plural form is inches. The plural possessive is inches'.
Coaster's is singular possessive. The plural is coasters, the plural possessive is coasters'.
The plural possessive is pals'.
The plural possessive of bandit is bandits'.