Noob question! Please entreat me!

[Brendanicus]

What happens to a summoned monster when they die? Say I use Summon Monster whatever to bring out a fiendish boar. Said boar receives the business end of a Paladin's smite before its time on the Prime was up. Does it die, or does it just return to its own plane?

[Shining Wrath]

Both. There will be a body there for an instant and then it will be gone.

[Brendanicus]

Both. There will be a body there for an instant and then it will be gone.
So it dies and vanishes? And leaving a corpse back on its own plane?

[BornValyrian]

No, it survives on its own plane. It doesn't make much difference unless you are using the summon unique creatures variant in the DMG. In that case if it dies you have to wait 24 hours to summon it again.

[Duke of Urrel]

My own personal understanding of Summoning spells – which does not have the authority of any official rule – is that they work like the Astral Projection spell, only backwards and instantaneously.

When you summon a creature from another plane, the creature's soul leaves its primary body behind in suspended animation. Immediately thereafter, the creature's soul forms a secondary body for itself on your plane, and this body appears exactly where you summoned it. However, the creature's soul is still connected to its primary body by a silver cord. If the creature is killed on your plane, its secondary body vanishes immediately afterward, but the silver cord whisks the creature's soul back to its primary body on its home plane. When the creature's soul re-enters its primary body, it awakens to life again on its home plane.

None of this applies to Calling spells, of course. When a called creature is killed, it dies in its primary body, so that it is really and truly dead.

[Duke of Urrel]

My own personal understanding of Summoning spells – which does not have the authority of any official rule – is that they work like the Astral Projection spell, only backwards and instantaneously.

When you summon a creature from another plane, the creature's soul leaves its primary body behind in suspended animation. Immediately thereafter, the creature's soul forms a secondary body for itself on your plane, and this body appears exactly where you summoned it. However, the creature's soul is still connected to its primary body by a silver cord. If the creature is killed on your plane, its secondary body vanishes immediately afterward, but the silver cord whisks the creature's soul back to its primary body on its home plane. When the creature's soul re-enters its primary body, it awakens to life again on its home plane.

None of this applies to Calling spells, of course. When a called creature is killed, it dies in its primary body, the only body that it has, so that it is really and truly dead.

[fluke1993]

I can't speak for the fluff but mechanically speaking; when you drop a creature summoned w/ a summon x spell bellow 1 HP it disappears instantly. The corpse being momentarily left behind is a fluff effect that probably comes about because death is rarely instantaneous. For example; if you get beheaded; you still have a couple of seconds to admire your gorgeous looking neck before you fade out.

Short of DM fiat, there is nothing you can mechanically do with the parts leftover before they disappear .

[Devils_Advocate]

A summoning spell instantly brings a creature or object to a place you designate. When the spell ends or is dispelled, a summoned creature is instantly sent back to where it came from, but a summoned object is not sent back unless the spell description specifically indicates this. A summoned creature also goes away if it is killed or if its hit points drop to 0 or lower. It is not really dead. It takes 24 hours for the creature to reform, during which time it can’t be summoned again.

When the spell that summoned a creature ends and the creature disappears, all the spells it has cast expire. A summoned creature cannot use any innate summoning abilities it may have, and it refuses to cast any spells that would cost it XP, or to use any spell-like abilities that would cost XP if they were spells.
- http://www.d20srd.org/srd/magicOverview/spellDescriptions.htm#summoning

[Crake]

My own personal understanding of Summoning spells – which does not have the authority of any official rule – is that they work like the Astral Projection spell, only backwards and instantaneously.

When you summon a creature from another plane, the creature's soul leaves its primary body behind in suspended animation. Immediately thereafter, the creature's soul forms a secondary body for itself on your plane, and this body appears exactly where you summoned it. However, the creature's soul is still connected to its primary body by a silver cord. If the creature is killed on your plane, its secondary body vanishes immediately afterward, but the silver cord whisks the creature's soul back to its primary body on its home plane. When the creature's soul re-enters its primary body, it awakens to life again on its home plane.

None of this applies to Calling spells, of course. When a called creature is killed, it dies in its primary body, the only body that it has, so that it is really and truly dead.

If that were the case, those creatures would be subject to a githyanki silver sword which could cut the chord, but there is no such thing. The creatures are physically summoned, and when they "die" they're sent back to their plane to reform from the planar matter of their home plane, which takes 24 hours.

[Brendanicus]

Also on the subject of summoning: If a Good Wizard summons Celestial creaures, and an Evil one summons Fiendish creatures, could a Neutral one summon either without consequence?

[Spore]

Also on the subject of summoning: If a Good Wizard summons Celestial creaures, and an Evil one summons Fiendish creatures, could a Neutral one summon either without consequence?

Yes, he can.

[Brendanicus]

Thanks for all the help guys. This new character idea I have is going to be great.