With regards to 5e, I wrote about this almost six years ago, but that was intended to be much less prescriptive than it came out.
Really I feel much the same about conversion guides as I do to monster design itself. I could try and give you a formula that lets you pull a monster straight from the 5e Monster Manual and use it with Bastionland, but it doesn't really get to the heart of how I see the place of monsters in the game.
There are things that are important in monster design when you're playing a game focused around tactical combat, where characters are most defined by their mechanical tools for interacting with this system of attack rolls, status effects, and grid-positioning. Bastionland isn't that sort of game, so if you want to transplant the 5e Chain Devil into your game, simply applying a recommended fraction to each of the numbers isn't going to result in an especially exciting creature.
We can make a method out of this. It's not something you can plug into a spreadsheet, but if you do it a few times it'll feel so much like second nature that you can just convert things on the fly.
It looks complex, but remember the goal is to reach a point where you don't need the list.
Bastionland Monster Conversion Guide
- Read over the information you have for the monster you'd like to convert. Get a real feel for the monster. Imagine what they're going to do in your game, where you might put them, what they might do to the players. Try to translate the numbers on the page into a living creature in your mind.
- Is it particularly strong or weak?
- If not, leave STR at 10. You don't even need to write this down, as it's the default.
- If particularly strong give it a STR around 14. If it's top-tier in this area then give it the full 18.
- If particularly weak, give it a STR around 6. If it's utterly deficient in this area then give it STR 3.
- Repeat the above for DEX and CHA.
- How good is this thing at staying alive? This isn't so much raw physique as fighting spirit.
- If it's not all that fighty then give it d6hp or pick a number in that range.
- If it's got some fight about it then give it 2d6hp or pick a number in that range.
- If this is a serious, boss-style creature, give it 3d6hp or any number up to 18 if you want it to be a seriously tough opponent to take down. Be careful here, as HP sponges can be a bit of a drag in practice.
- Does it have armour or other notable toughness?
- If so, start with Armour 1.
- Would you call it "heavily armoured?" If so, raise to Armour 2.
- Is it basically made out of armour? Like a robot or living statue? Armour 3.
- Any special immunities? Just make them immune to that sort of damage.
- If you don't want to go for full immunity, have certain types of damage be Impaired against them, but I'd urge you to go big when it comes to monster design.
- What's its most obvious form of attack?
- Pick a damage die between a sword (d6) and a naval cannon (d12).
- Can it reasonably target multiple targets? If so consider giving it two of these attacks.
- Can it reasonably target a whole bunch of targets? Give it Blast damage.
- Does it do something really nasty as a sort of "finishing move"? Have that effect occur when it causes Critical Damage.
- Does it have any weird offensive abilities that don't feel right as Damage?
- Would it be terrible if it could make them happen without even using dice? Just put it in as one of the creature's three "moves".
- Do you want it to be more unreliable? Give the target a Save to avoid.
- Does it have another weird ability that isn't really an attack? Just let them do it. Put it in their "moves".
- If this feels like too much (again I'd urge you to try it) then give it a certain restriction in its use.
Let's do that example
Chain Devil
Ability Scores: I'd say this guy is pretty strong (15) and dexterous (15). His CHA isn't really a factor.
Fighting Spirit: He's pretty serious, but not really a boss. I'd put him at the higher end of 2d6 (9hp)
Armour: He's covered in chains, so definitely counts as Armoured (1). I wouldn't call him heavily armoured.
Immunities: I know devils traditionally get a bunch of immunities, but without looking at the stat block there isn't anything that leaps out for this guy, so let's keep him as he is.
Attacks: He can throw his chains around. I wouldn't say he throws enough for a Blast, but definitely giving him two attacks. The chains feel like a step up from swords, so they can do d8 damage.
Finishing Move: Feels like there should be something here that we can tie into the traditional grapple effect of chains. On Critical Damage he starts to drain the target's soul, causing d12 CHA loss repeating each turn. When they reach CHA 0 their soul is utterly drained.
Other Abilities: It can animate chains, attacking with them as if it were holding them. I'll limit this to two chains in addition to its own two chain attacks.
Weird Abilities: So apparently it can change its face to resemble a loved one or bitter enemy. Don't really need any mechanics here, this is just a nice bit of horrible flavour for the monster.
We're done! I'll just throw in one extra move to give it a bit of a weakness.
Chain Devil
STR 15, DEX 15, 9hp, Armour 1.
Two chains (d8 each. d12 CHA loss on Critical Damage and every round afterwards unless the victim is freed. Drains their soul at CHA 0)
- Animates up to two chains (in addition to its own) that can lash out (d8)
- Changes its face to resemble a departed loved one or bitter enemy
- Withers to a husk if separated from its chains