The ranger and rogue in my game were asking about setting traps in preparation for a battle that was coming. Another player is convinced rogues can make traps because he saw it happen in D&D sessions online (in streams). But the only information about making traps that I can find seems to be addressed at DMs, in the context of designing dungeons and such.
Research
Rogues do get proficiency in Thieves' tools, but this only lets you disarm traps (not create them):
This set of tools includes a small file, a set of lock picks, a small mirror mounted on a metal handle, a set of narrow-bladed scissors, and a pair of pliers. Proficiency with these tools lets you add your proficiency bonus to any ability checks you make to disarm traps or open locks.
PCs are also able to buy Hunting Traps. So it would make sense that nifty rogues could backwards-engineer traps that they can buy and/or disarm, but I'm curious whether the books cover guidance in this regard.
Related:
- What options exist for player-controlled traps? (only covers "hasty traps")
- Unearthed Arcana: Traps Revisited
Question
What are the options for player characters to create traps?
By 'traps' I mean methods or mechanisms designed to influence, hinder, or expose enemies, which must be set up before use, operate on a trigger (set off by target or by trapper are both fine). For this question I'm excluding spells that function as traps.
Best Answer
Proficiency with thieves' tools, per optional rules in Xanathar's
Xanathar's Guide to Everything has a section on tool proficiencies that details what each tool set contains, a list of skill checks (for specific purposes) that proficiency with that tool grants advantage on, a possible special use of that tool for those who are proficient, and a short list of sample activities that can be performed with the tool along with suggested DCs for the necessary ability checks.
As you have reasoned, the special use listed for thieves' tools (per p. 84) is:
As this is quite open-ended, it's left to the player and DM to work out how exactly the trap works.
Roughly, the DM calls for an ability check to craft the trap (perhaps an Intelligence check with their proficiency bonus added, due to proficiency in thieves' tools - or double their proficiency bonus, if the character has Expertise with thieves' tools). That determines the DC to discover and/or disable the trap. The damage dealt by the trap is either half the total of the check (i.e. with a check of 16, it deals 8 damage) or some other number determined by the DM, with a damage type determined by the DM based on what sort of trap it is.
The snare spell
The snare spell (XGtE, p. 165) is a ranger, druid, and wizard spell that allows the caster to set a magic trap:
It takes a minute to cast, and lasts 8 hours. It also consumes its material component, which is 25 feet of rope. You probably want to combine this with another spell like alarm (or set up some kind of nonmagical alert system), so you know when a creature has triggered your trap. Otherwise, it may simply escape the trap within a few turns.