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In earlier editions of D&D some powerful creatures had a percentage magic resistance which was checked before any saving throw (if applicable). If the percentage check was successful then the spell simply didn't work.

I'm trying to convert the Dragonlance Draconians to 5e, but they had 20% magic resistance (and per the source materials this prevented Raistlin's sleep spell working on them).

Simply giving them advantage on saving throws won't work on magical effects without saving throws such as sleep, and ignoring the magic resistance completely in the conversion will significantly weaken the main antagonists (apart from dragons) in the Dragonlance Campaign.

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  • \$\begingroup\$ A global magic resistance. E.g. if a spell was cast on a creature there is a chance that spell just fails due to the creature having a percentage magic resistance. If it passes the magic resistance then spell acts as normal including any saving throws etc \$\endgroup\$ – Paul Hambly Sep 30 '18 at 7:27
  • \$\begingroup\$ I see this has been marked as duplicate, I looked at that answer and it doesn't answer my question as it only covers basic resistances allowing advantage on saving throws and my question goes into more detail asking how to convert Magic resistance from earlier editions that cover magic effects that don't require saving throws \$\endgroup\$ – Paul Hambly Sep 30 '18 at 7:31
  • \$\begingroup\$ I've edited the question to focus more on the conversion aspect \$\endgroup\$ – Paul Hambly Sep 30 '18 at 17:26
  • \$\begingroup\$ Looks like you got enough reopen votes. Let's see what answers you attract. Comments removed. \$\endgroup\$ – KorvinStarmast Sep 30 '18 at 18:13
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The mechanic you mention did not survive the transition to 5th Edition, as you've noticed. Most creatures now either just don't have the trait anymore, or have the trait that gives them Advantage on saving throws against magic. A few creatures are also flat-out immune to specific spells.

So the most straight-forward conversion would simply be to grant your creatures that trait and be done with it. There'll be a few weird edge-cases, but if you want simple and functional, that's the solution. If one of the specific spells like Sleep or Magic Missile bothers you, you might simply grant them immunity to those, to preserve the old feeling.

Alternatively, you'll need to come up with something yourself. The most straight-forward option is to simply do a 1-to-1 conversion of the old ability. That will work, although you'll have to explain it to your players, since it's outside the way spells normally work.

For what it's worth, I actually used creatures with an ability somewhat like this. I'll write it down here, in case the feature or the feedback I got from it are useful to you. These creatures were Living Spells and they could eat spells, but the ability should pretty much function like your idea of Magic Resistance while still being somewhat in line with the way 5e works if you just drop the healing part.

Devour magic

If the [living spell] is the target of a spell with a level of [x] or less, it rolls a d20. On a roll of 11 or better, the [living spell] is not affected by the spell at all and regains 1d6 hit points per level of the spell.

The spell level cap allows players to overwhelm magic resistance by using more resources (better spells) while the flat "11 or better" makes the ability more likely to matter (a 20% resistance likely never triggers in the average encounter).

As far as feedback goes, the main points are:

  • these things were very hard to kill due to their devour ability
  • spellcasters did not have an awful lot to contribute to this fight
  • the ability was initially very confusing to my players, because it's not an expected part of D&D 5e

So as I said; if you want to make this a common part of your world, definitely explain exactly how the mechanic works. Keep in mind it makes monsters that have it very tough to deal with, especially for spellcaster heavy parties and that excessive use will likely bore them out a bit. Having half your attacks fail with not much you can do about it, isn't particularly fun and because the mechanic isn't a core part of the game, there exists no way around it. (Unless you make one.)

It worked for me as a flavour-full and rare thing, but it did show me that this was not something I'd want to use more often than "rarely". Whatever you come up with as a conversion rule; those are probably the things you'll need to keep in mind.

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