Consequences of a Tumble and Overrun

Question

Page 272 of the DMG states:

Overrun

When a creature tries to move through a hostile creature's space, the mover can try to force its way through by overrunning the hostile creature

Tumble

A creature can try to tumble through a hostile creature's space, ducking and weaving past the opponent.

Both are optional Bonus/Actions a creature can make to move through another hostile creature. Now, my question is:

1.) Does a creature need to be 5 ft away from the hostile creature to do these actions?
What if there is a hallway 5 ft wide and a hostile creature battling a friendly creature that is positioned between you and the hostile. If you had enough movement to tumble/overrun, could you do it 10 or even 15 feet away, explaining it as a running start?

2.) If so, what is the consequence of failing the check?
More importantly, what square do you end up in on a failed Tumble/Overrun?

Answer 1

You are allowed to move through a space occupied by an ally, you just can't end your turn there. This means that you attempt to tumble or overrun when you are adjacent to the space you are trying to pass through. Remember that occupied spaces count as difficult terrain, so plan your movement accordingly. You still use up movement going through the enemy, and should have enough movement left to end up in an unoccupied space if you succeed.

If you fail to tumble or overrun, you would end up in the nearest unoccupied space. So in your example, you would end up behind your ally. It turns out that this general rule is not explicitly stated, however it is stated in every forced movement type of spell or ability, with no exception that I could find. If an action would normally place you into an occupied space, you instead get moved to the nearest unoccupied space. Since you never succeeded in moving into the enemy's space, the nearest unoccupied space is normally in the direction you just came from. If you have so many allies that the nearest unoccupied space is behind enemy lines, then it would be up to the DM to decide where you end up. If you are the DM, I recommend not allowing them to "succeed" in the overrun or tumble, even when they fail.

Answer 2

1.) Does a creature need to be 5 ft away from the hostile creature to do these actions?

An overrun or tumble attempt is made as part of a movement when you attempt to move through a hostile creature's space. As long as you have the movement to move through the creature (remembering that occupied spaces are difficult terrain) and end your turn in an unoccupied space, you can make a tumble/overrun check.

In the case that you give, you would move into your ally's space (temporarily) before attempting to overrun or tumble through the opponent. Once on the other side, you would still have your action to attack or do whatever you want.

2.) If so, what is the consequence of failing the check?

Undefined. There's no ruling as to what the effect of failing an overrun or tumble check is, but we can assume that because there's no stated rule, you simply aren't able to move through the occupied space but can continue your movement otherwise. This is subject to GM interpretation.

In your example, if you fail to pass through the enemy, you would have to backpedal to where you started in order to end your movement in an unoccupied space. If you can't end in an open space, it'd again be subject to GM interpretation (in previous editions you would drop prone, but 5e does not state that you can share a space with a prone creature).