In my current game, the PCs have been accompanied by a Toxic Skull (DFRPG Monsters, p. 27), aka a Skull-Spirit, for most of their recent sessions.
Back in the day, "skull-spirit" was synonymous with a hard fight or a quickly lethal foe. These days, not so much.
Let's look at the first version I know of, in GURPS Fantasy (SJG6001, 1986). I'm calling it 1e because it's the 1st edition, but it's possible this was written with reference to the 2e rules. I never saw or had those - I bought the GURPS boxed set as soon as possible. In either case:
Evil spirits, each created from the life-force of a human skull . . . A Skull-Spirit has a ghostly, vaporous form with ST 0, DX 14, IQ 10, HT 20. Speed 6. . . . strikes with a chilling touch which cannot be blocked or parried - it must be dodged. This touch does 2 hits of damage; armor is no protection. . .
The fight a skull-spirit, use Fire spells or Air Jet; other combat spells are ineffective. Magic weapons do normal damage. Swords, axes, and other large metal weapons do 2 hits of damage, arrows, bullets, etc. do 1 hit of damage. Beam weapons do no damage at all!"
I won't replicate the current version (look it up, buy the book!)
Here is what I see:
- It's interesting to see what's likely the very first published version of Diffuse (with a limitation: not versus magical weapons, Air Jet, or Fire spells.) What is a "magic weapon" isn't defined.
- HT 20 makes a big difference. It's where it gets its HP - 20. Not the lack of automatic death/destruction at -1xHP. These suckers could potentially go to -5xHP before destruction, and fail HT rolls on a 17-18. Instead of 40 HP to automatic destruction it's more like 120 points.
- Vulnerability to magic weapons. Undefined as to what those were - we always used, and still use, Puissance, but that's a house ruling - but there you go. You can bypass the maximum of 2 with a magic weapon.
- No special vulnerability to explosive spells.
- It's slow for its DX+HT; the modern version has the same speed, but twice the move and specifically defines it as Air move - the 1e version.
- Offense is the same. The type of injury is explained - and that it's injury, not damage (a more nebulous term in 1e/2e) is defined.
- the DFRPG version contains a lot more specified abilities - Doesn't Eat or Drink, Doesn't Sleep, etc.
- the DFRPG version is evil; the 1e/2e version doesn't make a judgment but does call it an "assassin." That's never a positive statement of morale rectitude.
On balance, the 1e/2e version is more vulnerable to well-equipped delvers but has a more HP, making it much harder to defeat if you aren't such a person. I think in my own games the guys who came up on the early version still have that "relentless, hard to kill, lethal" image in their head when it's more like "eventually lethal foe, unless you dump explosive spells on it" type of foe. And yeah, it might explain the occasional attempts to kill them with Flame Jet or Air Jet.