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Part of the description of the summon greater demon spell states (XGtE, p. 166):

If you stop concentrating on the spell before it reaches its full duration, an uncontrolled demon doesn't disappear for 1d6 rounds if it still has hit points.

The summon greater demon spell's duration is listed as "Concentration, up to 1 hour". A round in-game is about 6 seconds long.

If I stop concentrating on the summon greater demon spell 59 minutes and 59 seconds after casting it, could that extend the time the summoned demon is around for up to 35 seconds (assuming I roll a 6)?

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  • \$\begingroup\$ Just a small thing: rounds are approximately six seconds long so there is no 59'59" moment, only 59'54" \$\endgroup\$ – Medix2 Mar 31 '20 at 14:17
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    \$\begingroup\$ @NautArch It actually doesn't matter, because the demon doesn't persist for "1d6 * 6 seconds", but for 1d6 rounds, which is only approximately equal to 1d6 * 6 seconds. So yeah, if you roll a 6, the demon will be around for approximately 30 seconds longer than the normal duration of the spell. It might even be 35 seconds. \$\endgroup\$ – Mark Wells Mar 31 '20 at 15:34
  • \$\begingroup\$ @MarkWells That's an excellent point that should probably go in an answer :) I didn't think of it like that, but that definitely gives some direction for what to do. \$\endgroup\$ – NautArch Mar 31 '20 at 15:35
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You could get 5 additional rounds.

tl;dr The unit of concentration is a round. Breaking concentration in the final round gets an uncontrolled demon for up to 6 rounds including the one in which concentration was broken.

Breaking concentration immediately results in an uncontrolled demon.

The spell description details what happens when the demon is no longer controlled:

On a successful save, your control of the demon ends for the rest of the duration, and the demon spends its turns pursuing and attacking the nearest non-demons to the best of its ability.

The character concentrating chooses to break concentration on their turn during the last round of the spell's normal duration. If the demon's initiative is after the character, its is going to attack. If the demon's initiative was prior, it's already used it's turn. Either way the current round it is now uncontrolled.

Additional rounds.

The special case of ceasing concentration does add additional rounds:

If you stop concentrating on the spell before it reaches its full duration, an uncontrolled demon doesn't disappear for 1d6 rounds if it still has hit points.

Rolling a 6 when you cease concentration means that it does not disappear for six rounds. Since the round you cease concentration is the first round it is uncontrolled, this could net 5 additional rounds it remains beyond the duration of the spell.

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    \$\begingroup\$ Are we SURE that dropping concentration makes the demon "not controlled"? If you stop concentrating on the spell before it reaches its full duration, an uncontrolled demon doesn't disappear for 1d6 rounds if it still has hit points. could be meaning "if you still owned it, it disappears immediately. If this is you panic-dropping concentration because it broke free, then it sticks around to eat you" \$\endgroup\$ – vehementi May 24 '20 at 23:24
  • \$\begingroup\$ @vehementi: The previous sentences of the spell description, in conjunction with the sentence quoted earlier, confirm the only thing that causes a demon to be uncontrolled is passing the Charisma save: "At the end of each of the demon’s turns, it makes a Charisma saving throw. The demon has disadvantage on this saving throw if you say its true name. On a failed save, the demon continues to obey you. On a successful save, your control of the demon ends for the rest of the duration, and the demon spends its turns pursuing and attacking the nearest non-demons to the best of its ability." \$\endgroup\$ – V2Blast♦ May 25 '20 at 0:23
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No. Dropping concentration of a controlled demon causes it to disappear immediately.

A demon still under the caster's control disappears:

the demon disappears when it drops to 0 hit points or when the spell ends.

Only uncontrolled demons stick around for 1d6 rounds after you stop concentrating

Dropping concentration doesn't result in an uncontrolled demon. A summoned demon becomes uncontrolled only by succeeding on a saving throw:

On a successful save, your control of the demon ends for the rest of the duration

Only uncontrolled demons stick around:

an uncontrolled demon doesn’t disappear for 1d6 rounds if it still has hit points.

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  • \$\begingroup\$ I just noticed that you wrote both answers... Is there a reason for that? (Were you intending to present 2 different interpretations?) While I think this answer is generally right, the original question doesn't specify whether the demon is controlled or uncontrolled, so I don't think the answer to OP's question is "No"; rather, the answer is "it depends on whether it's still controlled or uncontrolled". (...But given that the demon would get up to 600 chances to break free of your control, since it can repeat the save every turn, it's almost certainly uncontrolled by the end of the duration.) \$\endgroup\$ – V2Blast♦ May 25 '20 at 0:29

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