[Mob] Skull of Sins Audit

[ThrisStraizo]

I affirm:

• I understand that if I do not agree to the following points by completing the checkboxes my PR will be ignored.
• I understand I should leave resolving conversations to the LandSandBoat team so that reviewers won't miss what was said.
• I have read and understood the Contributing Guide and the Code of Conduct.
• I have tested my code and the things my code has changed since the last commit in the PR and will test after any later commits.

What does this pull request do?

This PR audits the 7 Skulls of Sin for HP/Immunities/STP and general behavior.

https://youtu.be/0VNCHOjW1gU (will take a while to upload)

This also adds the missing gravity spell to the undead spell list, because it do be that way.

Steps to test these changes

Fight any of the Skulls of Sin and enjoy unrivaled retail accuracy.