Following are D&D 5E rules that can be used to adjudicate attempts by characters to votively replicate Hercules’ eighth labor, capturing and returning to Tiryns with the four man-eating Mares of Diomedes. This challenge takes place at Megalos Ellada, a large farm on the Peloponnesian Peninsula of mainland Greece and one of the places that appears in Skirmisher Publishing’s “In the Footsteps of Hercules” universal sourcebook. It is most suitable for characters of 1st to 4th level.

This is the fourth and last of the challenges characters can undertake over the course of a festival day at the farm and takes place after dark and with illumination by torchlight for thematic reasons. It is not necessarily very difficult, and is much less complex than the three that precede it — Riding the Bulls of Crete, Herding the Cattle of Geryon, and Stealing the Apples of the Hesperides — but the expectation is that characters may be injured, exhausted, and low on resources by the time they engage in it. 

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This event takes place at the farm’s theater and with the four “Mares of Diomedes” in one of the large bronze cages at the back of it. Once the gate to the paddock is opened, each participant must take one of the unbridled horses, lead it out, and guide it around the edge of the amphitheater’s performance area to an identical enclosure on the other side of it. If there are not enough characters present to handle all the horses then Alexander — one of the proprietors of the farm — will assign grooms to lead the remaining ones.

Anyone near a mare can make a Wisdom (Perception) ability check with a DC equal to the number of feet they are from it and, if successful, will detect a stench of rot and decay about it. Anyone within 5 feet of it can request to make an Intelligence (Investigation) check with a DC of 14 and, if successful, will be able to perceive the veneer of illusion around the creature and see its skeletal nature.

Leading a mare out of the cage can be accomplished automatically, but moving it to the halfway point of the semi-circular route requires a successful DC 11 Wisdom (Animal Handling) check. If a character fails it the horse will attempt to bite them. Then, at the midpoint of the route, each horse will make an attack against the person leading it regardless of success. If a horse draws blood, however, it will thereafter become more manageable, with Wisdom (Animal Handling) checks made against it being at advantage and any subsequent attacks it makes being half-hearted (i.e., at disadvantage).

At the storyteller’s option, when Fragiskos — master-of-ceremonies for the event and one of the proprietors of the farm — is narrating the story of Hercules and the Mares of Diomedes before the participants take part in the challenge, one of the people killed and eaten by the original monsters might have the same name as a party member. Anyone participating in this event who hears their name mentioned in such an inauspicious context will presumably be surprised and disquieted by it and must make a DC 15 Wisdom saving throw each round they are within sight of the mares. If they fail this check they will have disadvantage on attacks and ability checks until the beginning of their next turn. (In our own game one of the player characters actually was named Abderos, which was also the name of a companion of Hercules killed by the mares, and that inspired this twist.) 

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“Mare of Diomedes”

This horrible undead warhorse has been cloaked in an illusion similar in effect to that of Disguise Self, so that it appears to be a regal, powerful warhorse, and has been named for one of the legendary mares of Diomedes: Podargos, “the Swift”; Lampon, “the Shining”; Xanthos, “the Yel­low”; and Deinos, “the Terrible.”

If the necromancer Fragiskos is nearby, he will be able to control this monster and compel it to respond to things like attempts at Wisdom (Animal Handling) ability checks as if it were a normal animal. This will include allowing it to bite but not use its hooves against any characters attempting to interact with it. If its creator is not present, however, the mare will attempt to stomp to death a victim and then devour it, which will then sate and calm the monster for 24 hours.

“Mare of Diomedes”

Large undead, lawful evil

Armor Class: 11

Hit Points: 30 (4d10+8)

Speed: 60 ft.

STR             DEX             CON             INT               WIS              CHA

18 (+4)         12 (+1)         15 (+2)         2 (-4)            8 (-1)            5 (-3)

Damage Vulnerabilities: Bludgeoning

Damage Immunities: Poison

Condition Immunities: Exhaustion, Poisoned

Senses: Darkvision 60 ft., passive Perception 9

Languages: —

Challenge: 1 (200 XP)

Trampling Charge. If a “Mare of Diomedes” moves at least 20 feet straight toward a creature and then hits it with a hooves attack on the same turn, that target must succeed on a DC 14 Strength saving throw or be knocked prone. If the target is prone, the monster can make another attack with its hooves against it as a bonus action.

Calmed by Blood. If a “Mare of Diomedes” bites a character attempting to make a Wisdom (Animal Handling) against it, it will be somewhat sated and subsequent attempts will be at advantage. If the monster is injured after this happens, however, this effect will be dispelled.

ACTIONS

Bite. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 9 (2d4+4) slashing damage.

Hooves. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 11 (2d6+4) bludgeoning damage.