A single Stirge is little more than an inconvenience. Easy to hit, and rolling in with the same sort of HP as 1st level Magic User in 1st edition D&D; by which we mean 2 HP. Dealing with a Stirge is literally like swatting a fly, granted a very large scary looking one. A handful of Stirges becomes a little more problematic. 5 or so of them and you go from inconvenience to problem. A small swarm of Stirges is enough to take down a low level character. But if you really want to have some fun with Stirges they’re best used in large numbers! Any number above 20 and things get REAL fun! If you’re not careful you could find a swarm leaving characters dead, as in dead-dead. Not just making death saves, but friends hauling their desiccated and perforated body to a city looking for a high level cleric for hire. Every Stirge that hits attaches and continues to draw blood out it’s victim until it has drawn 10HP worth of blood. Once a character is down in 5e it goes into death saves, but any damage taken counts as a failed death save. So… if a character had 3 Stirges latched on when they go down… they’re likely headed to see that platinum dragon in the sky!
Stirges are a low CR creature that you should consider using in your game. Here’s 4 ideas for using them in your adventures:
1) Subsitute Stirges for mosquitoes: In the real world horses exist, in fantasy settings we have Unicorns. In the real world wolves exist, in fantasy settings we get Dire Wolves! It’s just fitting that fantasy mosquitoes are the size of house cats and regularly drain peasants dry! Any where that mosquitoes would exist in the real world is a good place to drop in Stirges! Replace the high-pitched whine of tiny wings with the terrifying flap of Stirge wings in your world!
2) Sewers! This is technically another mosquito substitution but one that’s easy to miss. Sewers are often portals to low level adventures. It’s there that we kill dangerous rats when we’re level 1, it’s there that we get paid to evict the pesky Kobolds at level 3. It’s there that Stirges can live, and breathe, and breed, and descend upon our brave adventurers! They have dark vision and a move speed of 40′ (which if you’re a nefarious DM is MUCH faster than your party since they’re knee deep in muck and water, aka: difficult terrain!) so the warm, smelly dankness is ideal for them.
3) Fantasy Malaria! Yet a third mosquito sub… In the real world mosquitoes carry all sorts of blood born pathogens! If you want to add a little bump to Stirges you could add a disease to the mix. Re-skin the Sewer Plague we’ve been gifted in the DMG and whenever someone gets bitten let the CON saves begin! This could also serve as a great story hook if you need one. Stirges are already deadly to commoners, now that they bring disease how much more deadly are they now? Maybe it’s up to your party to brave the swamps to recover a rare plant that is the key to curing the town.
4) TRAPS! Never forget: Kobolds are nasty, clever little creatures! What’s worse than a pit trap with a closing trap-door on it? a pit trap with a closing trap-door that also releases Stirges into the pit with whatever victim falls into it. What’s worse than a portcullis that falls and splits the party? A portcullis with a very tight cross bar pattern that splits the party AS small doors open releasing Stirges! In the words of some TV chef, “kick it up a notch!”
Hopefully these four ideas get your creative juices flowing! Stirges LOVE creative juices… but they love blood juices much more! May your party soon come to fear these flying parasitic pests!