Ah the Imp, The little lawful Evil 2 foot talk Extra Planer schemers. Wild little Devils of the least order. They do their masters bidding searching out souls to corrupt.  Most all imps are terrible cowards but that does not make them any less dangerous.
Imps can often be found in the service of powerful evil wizards.
Imps will only attack if they are sure that they can safely retreat or overcome their foe. Add to that most Imps will rely on their wicked stinger. Imps have a long scorpion like tail that can deliver powerful venom. Back in 1st edition this stinger was ridiculously brutal as a Save or Die attack, Yikes!  Later in 3.5 it was toned down but still very lethal with it being a Dex damaging attack.  In 4th it just became an ongoing poison effect. None the less deadly but far easier to get by then just falling over dead.  Now we see the Imp in its 5th incarnation and his sting deals a brutal 3D6 Damage ! Impressive.

Now as a Devil even a very minor one the Imp he enjoys all of the perks of his fellow Devil kind. Advantage on most damage attacks in 5E and a wide-set of DR in 4E and 3.5. Gives the annoying little creature the chance to slip away and let his poison do its work while running to the side of his Master.  And if you don’t get him in that one swing  you might get. He will probably get away to sting you another day.  As all Imps can become invisible at will.

Fun Plot Idea for a one night session.

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A cult on the outside of town, based in a near by mausoleum complex that is in search of a lost tome containing a summoning scroll to call on more powerful demons.
The town of Stolish (or any name in your game) is looking for volunteers to look into a string of arson in the city. Three buildings have been set fire in as many weeks. What are the connections between the three ?
Run this game as a mystery and give your players 3 clues.
Clue 1> Leads them to a local Thieves guild and proves wrong.
Clue 2> Leads them to a local  Wizard who recently had dealings with two of the three owners of the burnt buildings.  His house is the next to burn and he ends up murdered.
Clue 3>  A Scrap of Moss that grows only near old graveyards near swamps. Out of place for the local area.