Following on from the last post on my take on Alexis's amazing hp by mass idea, I think fleshing out my take on crawling on huge monsters is a natural follow-on.
The two main things that make my take on this unique are based on the way Spells and Steel treats damage and to-hit rolls.
In Spells and Steel, the to-hit roll doesn't represent the hard-to-hurt of a creature, but rather its ability to avoid being hit altogether. Therefore, when crawling on a creature, hits are automatic. If the creature has damage reduction, it still applies normally.
Movement on a creature is by Zones. Something like a horse would have just one Zone, while an elephant might have 2 or 3. A huge, 60' dragon might have 5 or 6. Only one person per zone. Make a Strength Save to move to the next Zone. Burglars may also use their freerunning abilities to move around on a creature.
Damage can be increased by crawling to a vital zone - for most creatures, this would be the head, but there's some weird beasties out there. For these purposes, if you can get to a high-value target, do max damage on each hit. Creatures with only one Zone always suffer max damage from someone crawling on them.
The other unique idea is this: for each round you spend on the creature, take 1 Fatigue. It's hard to hold on to a huge beast! This is obviously up to the GM's discretion - if it's something like a Sandworm from Dune, once you're on top, you're pretty much just standing on the ground, and taking a Fatigue would be silly.
If the creature spends its turn trying to get shake you off, take an extra Fatigue, and roll a Strength Save* to hang on. If you fail, you fall off and take at least 1d6 falling damage, (and maybe more, based on how far it is to fall...).
If the creature rolls over to crush you, roll a Strength Save to jump clear in time. If you succeed, you jump clear (but take at least 1d6 falling damage, more if appropriate). If you fail, take 1d6 damage per 500 lbs of creature. You can choose not to jump off, but instead brace a piercing weapon (spear, sword, etc.) against the creature. If you do, you'll take damage as if you failed your Strength Save, but you'll deal twice as much damage to the monster.
- When crawling on a creature, hits are automatic. DR still applies.
- If you are clinging to a vital area (i.e. head), do max damage automatically.
- Roll a Strength Save or use freerunning to move from Zone to Zone on the creature.
- When the creature tries to shake you, roll a Strength Save. Failure means you take at least 1d6 falling damage.
- When the creature tries to roll over and crush you, roll a Strength Save. Failure means taking 1d6 damage per 500 lbs of creature. Success means taking at least 1d6 fall damage.
- Or take the damage and brace a piercing weapon against the creature to deal twice as much damage as you take.