Recall Knowledge, on the other hand, I would argue is under-engineered. There's an awful lot of hand-waving going on there. Say you want to make a recall knowledge check to figure out a Clay Golem's abilities and weaknesses. What should the DM tell you if you roll a success? how about a critical success? Ask different DMs and you'll probably completely different answers. The "official" result should be success will give you their most prominent feature- which one is that? The spells that slow them? The ones that harm them? The ones that heal them? How about their cursed wounds? How about their saves? It's very vague and can make a major difference in a combat.
Again, spot on, Puggins!
Also note the three-trunked elephant in the room:
a) against any monster dangerous enough that Recalling Knowledge feels critical, you have less than a 50% chance of getting something out of a Recall Knowledge action. And I hope everybody agrees that for many monsters, a single "prominent fact" isn't enough to "solve" the puzzle that is said monster.
b) personally, I feel monsters are so dangerous in this game that even when I hand out a free monster knowledge check at the start of each combat, and hand out most pertinent info already on a single success, the monsters are
still very dangerous and very challenging.
and last but not least...
c) The opportunity cost. Each action spent on Recall Knowledge is 100% guaranteed an action that does not deal any damage, it hands out exactly zero buffs, and it is sure to debuff absolutely no enemy! The opportunity cost here is that just attacking is often sufficient to learn what you need to know. Even if you deal 15 damage less because of a vulnerability... you learnt that fact automatically for free while still (hopefully) dealing at least some damage.
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Running Recall Knowledge as I think I understand the RAW wants it to be run feels... overwhelming, unnecessary and like a hard difficulty upgrade.
No hero has time to spend more than maybe a single action on recalling knowledge when they're busy saving their own life. Even with five players, five such actions feels entirely insufficient to actually learn what you need to know.
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My point with c) is that in isolation an action spent on Recall Knowledge might seem like a fair deal. But when you factor in that this is not an action spent on actual combat, the total cost becomes overwhelmingly high in our opinion. Most combats are decided within three rounds. Spending even 1/9th on possibly failing to learn even a single thing is severely overcosted in my analysis. (It actually is worse, since you really need that information in the first round - and ideally before your melee bruisers act too! - for it to really matter)
Since the average cost of learning even a single thing is (more than) two actions, the result is that optimizing players simply learn about their foes the hard way, completely ignoring Recall Knowledge (until you get feats and spells that hand out checks for free).
Faced with that reality (and agreeing with the cost analysis) I started playing exactly the same way I'm playing 5th Edition: at the start of each encounter every player gets a free monster knowledge check, and even one success tells the party what they need to know. Much simpler for me, much more satisfying fun for the players - and combats are still plenty difficult, so noone feels anything of value is lost!
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If I have to constructively offer up a variant rule that retains as much of the Recall Knowledge structure as possible, I probably would go for:
You can make a Recall Knowledge check for free if and only if you take the basic Strike action. MAP applies to this RK check.
That is, the cost is now that you can't do your class-specific cool special attacks. But you still get to accomplish
something even when attempting a RK check. There is a cost (the difference between a "cool" attack and a "boring" one), but it is much more in line with the benefits. The MAP penalty ensures that spamming these checks is a losing proposition. (If you take something like the
Quick Draw action, you get a Strike as part of that action, but since you aren't taking the basic Strike action, no free RK check for you)
For spellcasters this becomes
You can make a Recall Knowledge check for free with every cantrip you cast to attack the creature you're attempting to recall info about.
That is, as long as you coast along with your cantrips (instead of letting loose with a slotted spell) you too get to contribute to the recall knowledge game.