So, best 3rd party adventures is tough…but I’ll go with M.T. Black’s Complete Adventures Vol. 1. It was tough because second place was Vol. 2. You should definitely buy both, Vol. 1 is more standard LMoP type stuff, but better, and lower level than Vol. 2, most all for below 5 which isn’t great cause there’s so much in this space, but doing the Harpell adventures from it would be a great intro to any Sword Coast set Big Book. Vol. 2 I think has better adventures and a bit higher level 5-10 stuff. All of these are top tier Sword Coast set things to do with your party, better than anything else published by anyone including WoTC, see reviews, deserve to be more famous.
Complete Adventures of M.T. Black Vol. I - This collection contains all of the M.T. Black adventures published in 2016. You get fourteen critically acclaimed and b
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For Monster Books I’ve read a ton of 3rd Party…the Level Up Monstrous Menagerie is by far the best monster book out there, especially for new DMs. But, problem is everyone has the regular MM and this has pretty much no new monsters, it does all the SRD stuff better than it’s ever been done, so if you’re a new DM, with a new group not already burnt on std. monsters, this is great for helping you make regular monsters awesome and meaningful and better than the regular MM. But it doesn’t have what most people want from a 3rd party monster book which is new monsters, with new tactics. I would love to see a Level Up style monster book with ALL NEW Monsters.
The Frog God books are real pedestrian, you should get 5e Foes for old school flavor monsters cause cheap, Tome of Horrors I and II are more old school flavor but boring monster mechanics.
Legendary Games Asian and Latin monsters are great thematic collections but like Frog God, boring mechanics and seem real shovelware, not as much as Tome of Horrors, but close.
The MM Expanded books are great for adding new, and surprising tweaks to experienced players, versions of standard monsters. If your party is fighting X type monsters again, these books are great for throwing a couple specials into the mix. But really, the new are not that inventive, it’s iterating on known stuff, the new art is awesome though,
Flee Mortals is shaping up to be yet another over promise under deliver product from MCDM, hope I’m wrong. But I guess minions might be cool, I’m seeing dissapointment though.
Then we have all the smaller press and dmsguild monster books…they’re mostly less known for a reason, but are good pickups if the niche appeals to you. I’d put up Twilight Fables as the best of this rest, but might be just cause it hit my need at the right time, still, it’s amateurish and awesome In the way a punk rock band’s first album is.
And so, not mentioned yet, all the Kobold Press books, which are exactly what you want for new inventive monsters with new inventive abilities. Tome of Beasts I-III and Creature Codex. All awesome, I forget which one has the Ogrepede but, man, when I had one burst from the ground mid boss battle my players minds were blown. Mostly, if you want a 3rd party monster book for new monsters with new havits, you want these books. Can’t go wrong with any of them.
And yet, the best 3rd party monster book is actually, The Monsters Know What They’re Doing. It’s got deep dives into the personalities of monsters and how they’d fight, which is a great reference, but as a whole, reading the whole thing and thinking on it teaches you how to get in the head of other monsters and explore all their possibilities. And this is will help you enjoy playing monsters more and thereby make your players more threatened by them And enjoying the fights.