Stirges are flying, bloodsucking parasites. One alone is a pest, but a flock of them constitutes a threat, and this is how they’ll nearly always be encountered.
Their Strength is pitiful, but their Dexterity is high. This makes them hit-and-run attackers, or rather hit-and-fly attackers, since they waddle along at a laughable 10 feet per turn but fly at a brisk 40. With a Wisdom of only 8, they’re indiscriminate in their target selection, attacking whoever comes closest to them; and with a barely cognizant Intelligence of 2, they know only one way to attack and stick with it regardless of circumstances.
This attack revolves around the use of their Blood Drain feature. Starting from a position in the air above its victim, a stirge dives down to bite with its mosquito-like proboscis. If it hits, it latches on. At the start of each subsequent turn, if its victim hasn’t yanked it loose, it automatically drains another 1d4 + 3 hp from him or her. Keep track of this number for each stirge individually, because once it’s drained 10 hp from its prey, a stirge is sated, and it detaches and flies away at normal speed (potentially incurring one or more opportunity attacks as it departs).
Note the wording of the Blood Drain feature: a stirge detaches after it drains 10 hp “or the target dies”—not, “or the target falls unconscious.” In other words, the stirge keeps slurping up its victim’s blood even after he or she falls to 0 hp. This means an automatic failed death saving throw on the part of the victim (“If you take any damage while you have 0 hit points, you suffer a death saving throw failure. . . . If the damage equals or exceeds your hit point maximum, you suffer instant death”—Player’s Handbook, page 197). These little blighters are tenacious—maybe fatally so, for entry-level player characters.
What if a stirge misses its attack? Then it flies back up into the air, holding station about 20 feet above its target, and tries again on its next turn. That’s its standard attack cycle: fly down, attack, then either latch on (on a hit) or fly back up (on a miss).
Stirges aren’t bright, but they have just enough savvy to know that it gets less tasty blood if it has to share a victim. Thus, if the nearest potential target already has another stirge attached to it, a stirge will pass it by and attack the second-nearest target. Only when other stirges have latched on to every other living being in the vicinity will they begin to double and triple up—half an artery is better than none.
Stirges are evolved creatures with a normal survival instinct, but they’re also very fragile: any damage sufficient to seriously injure a stirge is also sufficient to kill it outright. Thus, retreat isn’t something you need to concern yourself with when running a stirge encounter—at least, not on the part of the stirges. They’ll definitely pursue opponents who try to retreat, though, unless and until they’ve slaked their thirst for blood. Even though any one stirge may be well fed, every other stirge in the flock is going to want its share, too.
Note that stirges have darkvision, indicating that they’ll be encountered primarily at night or underground. They gain nothing from hunting in daylight. Consider them nocturnal.
Next: phase spiders.