Every year on January 1st, for over 15 years, my group of friends have been doing a "New Year's RPG session". I've played and DMmed my fair share of 3E, 3.5E, and D20 Modern, but I've stopped playing about 4 years ago. My friends have kept on playing and now prefer 5th edition.
This January 1st, I volunteered to DM a 5th edition "one shot". My question is simple (though possibly a tad bit broad):
What are the very very basic pieces of rules I should know about when DMming 5th edition?
To make it less broad, assume:
- Someone reasonably experienced at DMming (including "winging it" if needed)
- A fully fleshed out, custom, home-brew one-shot story ready to go (backgrounds, maps, etc all taken care of by me)
- Helpful, cooperative players, several of whom have DMmed 5th edition themselves
- Near-zero risk of power playing, rule abuse, and the like from players
- Level 5 characters
- Adventure with intended mix of combat, skills, puzzles, and role playing
It's hard to "know what you don't know", but judging from earlier editions, I'm particularly worried about things like these from 3e and 3.5e:
- Things like passive abilities (like ones to detect secret doors) I should know about
- Certain skills being "opposed", where the DM should roll for a player, hiding the result
- What reasonable DCs are (e.g. what a reasonable AC would be, or a reasonable break DC for a door)
- Combat rules that you'd really want to read up on, ahead of time (e.g. Grapple was so nontrivial, that if you have a grappling enemy in the story you'd better read up in 3.5e)
My players have all the books, and they've also provided me with the 5th ed. SRD (pdf), but I have neither time nor interest to read through it all (right now, at least).
Best Answer
For a DM well versed in previous editions, I would highlight the following:
Setting DC and AC
Bonuses to skill checks and AC do not increase much as characters level, thus AC/DC "grades" can be assumed to be constant.
low/medium/high AC: 12/16/19
basic/challenging/hard DC: 10/15/20
Any kind of target number over 20 will be quite a hurdle even for high level characters.
Passive rolls
There are only two skill you have to look out for here: perception and insight. The former taking the place of search and notice, the latter of sense motive. What previous editions called "take 10" 5e calls a passive check, with the value unchanged. Rolls made to hide or lie are usually made against the appropriate passive check. You should write down these values for PCs and NPCs alike.
New mechanics
Advantage
Situational modifiers have been practically compressed into the advantage/disadvantage mechanic. You roll the d20 twice and take the higher or lower result as appropriate. Note that this does not change the range of the possible results and thus supports the static AC/DC scales. Giving a +x based on circumstances is not advised.
Concentration
Stacking spells is also very limited and thus the bonuses gained are constrained. Most buff and debuff spells require concentration and a single character can only concentrate on one spell at a time. Concentration can be broken by dealing damage to the caster (DC10 Con save to keep it in nearly all cases if your PCs are level 5).
Combat
The rules are much more streamlined in 5e. You should read the list of conditions in the PHB, starting on p.290 and spanning a bare 3 pages with copious amounts of illustrations. Other minor things: