What happens it the trigger becomes impossible. For example, if the trigger is “when I die” and I later become immortal and invincible, does the spell end?

The spell is labeled as "Until Dispelled" which can be done whenever the heck you want. Also if the creature takes damage from any source the spell ends. 

So I can permanently sequester something?

Wait I missed the part where it says that can set a trigger. I thought you had to set a trigger.

This spell might be one of the most powerful divination spells, even though it's a transmutation. 

Get 3 pitons and 1 ball of iron, then cast sequester on the ball and make the condition "when there is a  X  within  a one mile cube.

Next cast 3 sequesters on the pitons. make the condition "when the point of the piton is pointed at X "

Now you can fly all over the place and find whatever your looking for.

I would say not, as the spell can be ended by the target simply taking damage or sequester being dispelled, meaning there are still methods for the spell to end.

If you use this on a summoned creature  would it extend the summons duration? Also could you use this on glyphs to hide them? Having an invisible weapon on your person can be useful as well

Quote from TheGodTree >>

If you use this on a summoned creature  would it extend the summons duration? Also could you use this on glyphs to hide them? Having an invisible weapon on your person can be useful as well

 not really shure if the magic glyphs from the symbol and glyph of warding spells are themselves considered an object, since they are typically made of powdered iron or diamond or not even coporeal. It would probably not extend the duration of summoning spells, and the summoned creature would just dissapear for you, going back to its home plane in the state of suspended animation you left it in

also why is this transmutation? it makes way more sense as an abjuration or illusion spell based soley on how it works

Quote from ArtificeMeal >>

also why is this transmutation? it makes way more sense as an abjuration or illusion spell based soley on how it works

 I'm curious some on the opposite question: if it being Transmutation were set in stone, what could be changed about the manner it works to make it more befitting of the school?

Quote from ArtificeMeal >>

also why is this transmutation? it makes way more sense as an abjuration or illusion spell based soley on how it works

 If it were Abjuration, it would need a warding effect, such as preventing damage or removing the target from the current plane; if it were Illusion, it couldn't have the suspended-animation effect. Sequester being a Transmutation spell allows it to alter the state of the target - in this case, stopping the passage of time.

Notably, nothing here seems to stop the target from still being interacted with still.

Find some non-divination way to see invisible, could find forgotten Sequestereds and drag them 'round, even all the way to somebody who could dispell it if you cannot.

Could be plot thing.

Quote from DIemortalscum >>

This spell might be one of the most powerful divination spells, even though it's a transmutation. 

Get 3 pitons and 1 ball of iron, then cast sequester on the ball and make the condition "when there is a  X  within  a one mile cube.

Next cast 3 sequesters on the pitons. make the condition "when the point of the piton is pointed at X "

Now you can fly all over the place and find whatever your looking for.

It occurs to me that you could do something similar to create 5e GPS. If you cast 3 sequesters on 3 objects, with your original condition of "When X is within 1 mile of object" and then give them to 2 buddies and fly around a few miles apart. Assuming what you're looking for is a non moving thing such as a temple, you could triangulate a rough position.

While that's a cool idea, the main issue that comes up is the 5,000 gp cost to cast the spell. Casting it three times is doable, but just for the purposes of locating an object, it better be worth it.

can you cast this on your house and if so would you be undetectable within it?

So this doesn't actually specify that you're unconscious. It only says suspended animation. And even if you argue that suspended animation means unconscious, what happens when it is cast on a creature that is immune to the unconscious condition? If you're not unconscious, this is basically free immortality and invisibility to all divination. The perfect way to hide.

Quote from Joesaurus >>

So this doesn't actually specify that you're unconscious. It only says suspended animation. And even if you argue that suspended animation means unconscious, what happens when it is cast on a creature that is immune to the unconscious condition? If you're not unconscious, this is basically free immortality and invisibility to all divination. The perfect way to hide.

 Not really since the very next sentence is:

>Time ceases to flow for it, and it doesn't grow older.

Which tells you that it's not merely the Unconscious condition. 

Quote from DIemortalscum >>

This spell might be one of the most powerful divination spells, even though it's a transmutation. 

Get 3 pitons and 1 ball of iron, then cast sequester on the ball and make the condition "when there is a  X  within  a one mile cube.

Next cast 3 sequesters on the pitons. make the condition "when the point of the piton is pointed at X "

Now you can fly all over the place and find whatever your looking for.

 That would be an extremely expensive use of this spell, since each casting costs 5,000 gp and ending it early would mean you'd have to reset it. It seems like it'd be more practical to use an array of magic mouths instead.

Quote from kjart >>

can you cast this on your house and if so would you be undetectable within it?

 It would not work on a house as the spell says. "By means of this spell, a willing creature or an object can be hidden away, safe from detection for the duration." creature or an object.
The house isn't a creature so maybe it's an object so it would work? Sadly it does not.

In the basic rules chapter 15 it has the rules of objects stating: "For the purpose of these rules, an object is a discrete, inanimate item like a window, door, sword, book, table, chair, or stone, not a building or a vehicle that is composed of many other objects."

Therefore you couldn't sequester a house but if it had one entrance then perhaps a door would work. Other options for being undetected personally are the spells nondetection and mind blank. Or look at the spell private sanctum to do an entire area up to 360,000sq feet. 

i cant tell if this is good or not