Demolitionist

Most rogues prefer to keep a low profile, treading carefully amongst the cover of darkness or leaping silently across rooftops. Some however, prefer to keep things loud and dangerous. Demolitionists are rogues who excel in this field, being bonafide experts at making things explode - no matter if it's a bank’s vault door or a group of enemies.

Using their expertise in alchemy and volatile reagents, combined with a good sense for pinpointing structural weaknesses, these rogues also make excellent saboteurs - reducing even the strongest of defenses to rubble with their explosives.

Blastmaster

Beginning when you choose this archetype at 3rd level, you’ve learnt how to craft potent explosives. You gain proficiency with Alchemist's supplies. As part of a long rest you can use a set of Alchemist's supplies to craft a number of explosives equal to your proficiency bonus + 2. Any explosive you craft is rendered inert and useless whenever you finish a long rest. As an action, you can set one of these explosives to detonate in one of the following ways:

Thrown. You set the explosive to detonate on impact and throw it at a point within 30 feet of you. 

Trigger. You set the explosive to detonate upon detecting a creature coming within 10 feet of it, using a length of wire, a pressure plate or an arcane rune.

Fuse. You set the explosive to detonate when a certain amount of time has passed. Choose a number of rounds, minutes or hours up to your proficiency bonus. Once the chosen time has been reached, the explosive detonates. If you are in combat when the detonation occurs, it happens on initiative count 0 or 20 (your choice).

When one of your explosives detonates, any creature within 10 feet of it must make a Dexterity saving throw against a DC of 8 + your proficiency bonus + your Intelligence modifier. On a failed save, a creature takes 3d6 fire damage. On a successful save, a creature takes half damage.

Whenever you set a Trigger or Fuse explosive, roll a Dexterity (Sleight of Hand) check and record the result. Any creature must then have a Passive Perception equal to or higher than the recorded result to notice the explosive. If a creature had line of sight to you when you set the explosive, it can instead roll a Wisdom (Perception) check contested by your Dexterity (Sleight of Hand) check, noticing it on a success. When a Fuse or Trigger explosive detonates, you can apply your Sneak Attack damage to the explosives damage roll against a creature that did not notice it before it detonated.

The damage dealt by your explosives increases by 1d6 when you reach 6th level (4d6), 9th level (5d6), 13th level (6d6) and 17th level (7d6).

Professional Saboteur

At 9th level, you're an expert at laying traps and sabotaging defensive positions from behind enemy walls. You gain proficiency with Disguise Kits, and have advantage on any Charisma (Deception) checks you make to maintain a disguise, as well as on Intelligence (Investigation) checks made to pinpoint structural weaknesses in buildings and structures.

Controlled Detonations

Also at 9th level, your experience with explosives allows you to prevent collateral damage. Whenever an explosive crafted through your Blastmaster feature detonates, you can designate a number of creatures up to your Intelligence modifier within the blast radius. The designated creatures automatically succeed on their saving throw.

Elemental Charge

At 13th level, you can modify the energy unleashed when your explosives detonate. Whenever you set or throw an explosive crafted through your Blastmaster feature, you can choose to have it deal one of the following damage types, instead of fire damage; acid, cold, lightning, or poison damage.

Devastating Demolitions

At 17th level, your explosives are potent enough to bring down even the sturdiest of defenses. Your explosives deal double damage to buildings, objects and surprised creatures.