If a paladin is mounted on his summoned horse (via Find Steed spell) and the mount gets killed, what happens to the rider? Does he follow the rules for when the mounted creature drops prone?
The rider should follow the rules of the mount getting knocked prone
There isn't anything in the core rules that covers this exact situation, but a common-sense approach should be enough to make a ruling.
The Find Steed spell says:
When the steed drops to 0 hit points, it disappears, leaving behind no physical form.
The rules on mounting and dismounting state:
If your mount is knocked prone, you can use your reaction to dismount it as it falls and land on your feet. Otherwise, you are dismounted and fall prone in a space within 5 feet it.
If you combine the two, and consider that instead of going prone the mount has simply disappeared, allowing the rider to make the reaction dismount seems reasonable. If they chose not to use their reaction, then they fall prone.
A reasonable DM may decide to just call for a Dexterity saving throw from the rider as well, landing on their feet on a success, or falling prone in the dirt on a failure. But I think the rules lean more towards the dismount using a reaction. The rules on mounting and dismounting mention this:
If an effect moves your mount against its will while you're on it, you must succeed on a DC 10 Dexterity saving throw or fall off the mount, landing prone in a space within 5 feet of it.
But in this case, the situation is not the same. The save is to fall off or stay on the mount, which clearly isn't an option in this case where the mount doesn't exist any more.
While there's nothing specifically about this exact situation in RAW, I think looking at the chain of events would help:
- Steed has 19hp
- Steed is hit with 19 damage
- According to the Dropping to 0 HP rules: "If damage reduces you to 0 hit points and fails to kill you, you fall unconscious. This unconsciousness ends if you regain any hit points."
- According to the rules for Unconscious: "The creature drops whatever it's holding and falls prone."
- According to the rules for Mounted Combat: "If your mount is knocked prone, you can use your reaction to dismount it as it falls and land on your feet. Otherwise, you are dismounted and fall prone in a space within 5 feet it."
- Then, according to the rules for Find Steed: "When the steed drops to 0 hit points, it disappears, leaving behind no physical form."
The key is that the rules for Dropping to 0 HP still take effect before those about the steed disappearing. If that sounds a bit strange to you, think about all the effects that trigger when something dies and how it would be a bit weird if immediately disappearing prevented those from triggering.
If the Paladin dismisses the steed, then the steed doesn't fall prone and the Paladin is automatically left standing.