I like to make monster knowledge an important factor in combat in order to reward characters that focus on knowledge and to encourage a variety of tactics. At the same time, I want a world full of mysterious/unknown threats that may throw the players for a loop until they figure out how it works ("puzzle monsters"). So I treat knowledge checks not just as what you know, but what you can deduce from observing a creature's behavior and actions. I break monster knowledge into 4 tiers that require progressively higher skill DCs and/or action usage, and the longer the fight goes the easier it is to get info.
Upon sight (or when initiative is rolled), players with the relevant proficiencies receive basic info that covers origin/type and any obvious or well-known traits. If these were trolls one day from town, fire/acid effects would be here. If trolls were rare or avoided people in the setting, the info would not be here.
PCs can also spend a bonus action or action making checks to further observe and receive more info such as high/low stats or resistances/vulnerabilities. If they spend a whole action, I try to give them at least one exploitable weakness to make the action "worth it" in terms of action economy. In most of my settings, the beholder example would be in the 3rd or 4th tier of info because they are rare and very few people have seen them and survived to tell the tale.
I also encourage and emphasize general traits that apply to a creature type, like if the party is going up against undead they can expect that dealing radiant damage will be beneficial as would having resistance to necrotic. I like the idea that you can take what you've learned from past fights and apply it to future ones, and my players really respond to that.
My group is definitely prone to metagaming, and everyone knows monsters because we all DM to some extent, so my games generally have about 90% custom monsters. I like doing this anyway, but if I didn't I could understand the frustration some people might have with players knowing everything.
I like to make monster knowledge an important factor in combat in order to reward characters that focus on knowledge and to encourage a variety of tactics. At the same time, I want a world full of mysterious/unknown threats that may throw the players for a loop until they figure out how it works ("puzzle monsters"). So I treat knowledge checks not just as what you know, but what you can deduce from observing a creature's behavior and actions. I break monster knowledge into 4 tiers that require progressively higher skill DCs and/or action usage, and the longer the fight goes the easier it is to get info.
Upon sight (or when initiative is rolled), players with the relevant proficiencies receive basic info that covers origin/type and any obvious or well-known traits. If these were trolls one day from town, fire/acid effects would be here. If trolls were rare or avoided people in the setting, the info would not be here.
PCs can also spend a bonus action or action making checks to further observe and receive more info such as high/low stats or resistances/vulnerabilities. If they spend a whole action, I try to give them at least one exploitable weakness to make the action "worth it" in terms of action economy. In most of my settings, the beholder example would be in the 3rd or 4th tier of info because they are rare and very few people have seen them and survived to tell the tale.
I also encourage and emphasize general traits that apply to a creature type, like if the party is going up against undead they can expect that dealing radiant damage will be beneficial as would having resistance to necrotic. I like the idea that you can take what you've learned from past fights and apply it to future ones, and my players really respond to that.
My group is definitely prone to metagaming, and everyone knows monsters because we all DM to some extent, so my games generally have about 90% custom monsters. I like doing this anyway, but if I didn't I could understand the frustration some people might have with players knowing everything.
My homebrew subclasses (full list here)
(Artificer) Swordmage | Glasswright | (Barbarian) Path of the Savage Embrace
(Bard) College of Dance | (Fighter) Warlord | Cannoneer
(Monk) Way of the Elements | (Ranger) Blade Dancer
(Rogue) DaggerMaster | Inquisitor | (Sorcerer) Riftwalker | Spellfist
(Warlock) The Swarm