Alchemist's Fire is a kind of Adventuring Gear which you can throw to deal 1D4 points of Damage to a creature and set it on-fire, dealing 1D4 damage at the start of each of its turns. From Roll20 (and the SRD):
Make a ranged Attack against a creature or object, treating the alchemist's fire as an improvised weapon. On a hit, the target takes 1d4 fire damage at the start of each of its turns. A creature can end this damage by using its action to make a DC 10 Dexterity check to extinguish the flames.
Using a Crafting Skill, what would happen if a Character took a regular arrow and attached a vial of Alchemist's Fire to it, say with sinew, then a Rogue fired it from a bow?
Could the Rogue then use this to Assassinate and Sneak Attack a Target? Would a Critical from the Assassinate effect (Surprised Target) also apply to the Fire Damage, or would that be exempt?