Said game also doesn't have the Arcane Trickster archetype, so I was okay letting this be a little more complex to take up that design space.
I took the Survivalist feature directly from the published Scout archetype.
Also includes a rules for grenades.
SurvivalistWhen you choose this archetype at 3rd level, you gain proficiency in the Nature and Survival skills. Your proficiency bonus is doubled for any ability check you make that uses either of those proficiencies.
TrapsTrapper's Kit: You have assembled a trappers' kit. It is a unique combination of wires, vials, blasting caps, strings, acids, and other interesting odds and ends you've "acquired" in your travels. In conjunction with thieves' tools, it allows you to improvise traps.
The trapper's kit has 4 uses. Whenever you take a short or a long rest, you're able to repurpose and cannibalize portions of it and your environment to restore all expended uses.
Your kit gains another use at 9th and 17th level.
Trap Proficiency: Starting at 3rd level, add your proficiency modifier to any attacks by, saving throws against, or checks to detect or escape from traps you set. Ball bearings, caltrops, grenades, hunting traps, and nets count as traps.
You can use the bonus action granted by your Cunning Action to Ready, but only to respond to a creature you can see taking damage from a trap you set. If this trigger occurs, you can use your reaction to add half your sneak attack dice to the trap's damage (round up).
Additionally, you can use this bonus action and 1 use of your kit to produce a simple trap which emulates a bag of ball bearings, caltrops, a grenade, a hunting trap, or a net. These aren't durable and cease to be effective after 1 minute.
Improvised Traps: Choose 2 of the following improvised traps you're familiar with.
By spending 1 minute, 1 use of your trapper's kit, thieves' tools, and items scrounged from your surroundings, you set one of the chosen traps.
You can reliably improvise a number of traps equal to your Intelligence modifier (minimum 1). When you set another, one of the existing ones, determined at random, fails and becomes harmless.
You can choose another improvised trap to be familiar with at 9th, 13th, and 17th level.
The DC to avoid, disarm, or detect any of these trap effects is 8 + your proficiency bonus + your Intelligence modifier.
Improvised TrapsAlarm: When you set this trap, choose if it will be subtle or blaring. When tripped, you automatically hear the subtle version it within 60 feet, but anyone else must make a Wisdom (Perception) check against the trap's DC to tell they have tripped it. The blaring version is audible up to a mile away outdoors.
Bomb: Each creature in a 10-foot-radius sphere centered on the triggering creature must make a Dexterity saving throw. They take 4d6 fire damage on a failed save, or half as much damage on a successful one. Audible up to a mile away outdoors.
Dart: The trap makes an attack against the triggering creature, adding your proficiency bonus and your Intelligence modifier, for 2d6 damage. Extremely quiet.
Flash: A blinding light flares in a 20-foot-radius sphere centered on the triggering creature. All within must pass a Constitution saving throw or be blinded. They can attempt a new saving throw at the end of each of its turns.
Release: A container rigged to open or break when the trap is sprung. You must provide something to fill it with. A pint will affect the triggering creature. A gallon will usually affect anything within 10 feet of the triggering creature.
Common choices include: Ball bearings, resin, acid, grease, holy water, bees, scorpions, caltrops, confetti or glitter, stinky muck.
Snare: The triggering creature must pass a Dexterity saving throw or be restrained. It can use its action to make another Dexterity saving throw at disadvantage to escape. You may opt to construct this trap such that it deals 1d6 damage when it initially restrains a creature.
TrailblazerStarting at 9th level, moving through nonmagical difficult terrain costs you no extra Movement.
In addition, while traveling for an hour or more in the wilderness, your group moves more effectively under your leadership:
- Moving at a fast pace does not impose a penalty to passive Wisdom (Perception) scores.
- Your group can use stealth at a normal pace.
- Moving at a slow pace allows your group to use stealth and imposes disadvantage on any Wisdom (Survival) checks made to track you and them.
Urgent ImprovisationAt 13th level, you can create an improvised trap as an action rather than in a minute by spending twice its uses from your trapper's kit.
Chain ReactionStarting at 17th level, if a creature takes damage from or fails a saving throw against a trap you set, your first attack next round against it has advantage.
Simple TrapsThese are provided for reference. Add your proficiency bonus to any of these traps' DCs when you use them, and to any attack roll you make with them.
Ball Bearings: As an action, you can spill these tiny metal balls from their pouch to cover a level, square area that is 10 feet on a side. A creature moving across the covered area must succeed on a DC 10 Dexterity saving throw or fall prone. A creature moving through the area at half speed doesn’t need to make the save.
Caltrops: As an action, you can spread a single bag of caltrops to cover a 5-foot-square area. Any creature that enters the area must succeed on a DC 15 Dexterity saving throw or stop moving and take 1 piercing damage. Until the creature regains at least 1 hit point, its walking speed is reduced by 10 feet. A creature moving through the area at half speed doesn't need to make the saving throw.
Grenade: As an action, you can throw a grenade up to 60 feet. Make a ranged Attack, modified by cover, treating the grenade as an improvised weapon. At the start of your next turn, the grenade explodes. Everything within a 10-foot-radius centered on it must make a DC 13 Dexterity saving throw. They take 2d6 fire damage on a failed save, or half as much damage on a successful one. A creature can use its action to attempt a Dexterity saving throw with a DC equal to your attack roll to pick up the grenade and throw it back. On a failure, it explodes immediately, and they cannot attempt a saving throw for half damage.
Hunting Trap: When you use your action to set it, this trap forms a saw-toothed steel ring that snaps shut when a creature steps on a pressure plate in the center. The trap is affixed by a heavy chain to an immobile object, such as a tree or a spike driven into the ground. A creature that steps on the plate must succeed on a DC 13 Dexterity saving throw or take 1d4 piercing damage and stop moving. Thereafter, until the creature breaks free of the trap, its Movement is limited by the length of the chain (typically 3 feet long). A creature can use its action to make a DC 13 Strength check, freeing itself or another creature within its reach on a success. Each failed check deals 1 piercing damage to the trapped creature.
Net: A Large or smaller creature hit by a net is Restrained until it is freed. A net has no effect on creatures that are formless, or creatures that are Huge or larger. A creature can use its action to make a DC 10 Strength check, freeing itself or another creature within its reach on a success. Dealing 5 slashing damage to the net (AC 10) also frees the creature without harming it, ending the effect and destroying the net. When you use an action, Bonus Action, or reaction to Attack with a net, you can make only one Attack regardless of the number of attacks you can normally make.