Noob question

Sorry in advance if this is too silly of a question to ask, kinda new player here, started playing after Arcane though I had tried the game before, mainly a HS player. 

I was playing versus a Shurima deck with my Trundle/Aurelion Sol deck. I didn't want to use my 6 mana daybreak removal (6 damage and silence one) because I knew he can place his Azir in the stasis thingie, Ancient Hourglass I think is the name. So feeling smart I discovered (invoked) the 6 mana obliterate a unit or landmark and expected it to work ... But it didn't. Question is: Why? I know effects work in reverse play order, so Azir should be placed in the stasis first, but being a landmark it should still be destroyed by the spell, no? 

Edit: If this is considered a better fit for General forum and less of a card discussion, please move my thread. Seemed more like a card question to me.

it is a slow spell you cant use it in response to a skill or spell you have to play it as the  your first action  of the round.

You were totally reasonable to think that Falling Comet would work there. I actually would have expected that to work as well, but I think I know now why it failed. I also think I know why they worded and coded Ancient Hourglass and Entomb the way they did. So, let me explain:

Both of these spells have the same wording, and both of them replace a unit with a landmark. However, they do not transform the unit into a landmark. They obliterate the unit and then summon a totally new landmark that holds a new unit with identical stats to the original unit.

Here's the key - when a unit is obliterated, it is completely and utterly removed from the game. You can never get it back in any way shape or form, even with revive effects. It's just completely gone. So, you targeted a unit that was on the board, but that unit got entirely obliterated from the game. It was not transformed into the landmark. So, when the obliterate happened, your spell's target disappeared. It was the same effect as the unit being killed off and not replaced.

This is why the spells use the Obliterate keyword - they automatically negate any spell that targets the unit they protect. (Though the opponent could still respond afterwards with a fast or burst spell, as always ... but the original spell will always be negated by either of these protective spells.) Note - I've been playing the game for about a year and a half, and I actually just learned something by thinking through this question.

TL;DR - The target of your spell was obliterated, so your spell no longer had a target and therefore fizzled.

Sto already guessed what happened here.

Spells always target the current instance of a unit. By the unit leaving play in any way before resolving the spell the spell loses it's target and fizzles. 

In mtg we have a comparable, more widely used mechanic called flicker. It does obliterate a unit to summon the unit again which in mtg means all instances targeting the creature before fizzling. 

In LoR exactly the same thing is happing: if a unit leaves the battlefield before your spell resolves, your spell's targeted effect is gone

Quote From sto650

You were totally reasonable to think that Falling Comet would work there. I actually would have expected that to work as well, but I think I know now why it failed. I also think I know why they worded and coded Ancient Hourglass and Entomb the way they did. So, let me explain:

Both of these spells have the same wording, and both of them replace a unit with a landmark. However, they do not transform the unit into a landmark. They obliterate the unit and then summon a totally new landmark that holds a new unit with identical stats to the original unit.

Here's the key - when a unit is obliterated, it is completely and utterly removed from the game. You can never get it back in any way shape or form, even with revive effects. It's just completely gone. So, you targeted a unit that was on the board, but that unit got entirely obliterated from the game. It was not transformed into the landmark. So, when the obliterate happened, your spell's target disappeared. It was the same effect as the unit being killed off and not replaced.

This is why the spells use the Obliterate keyword - they automatically negate any spell that targets the unit they protect. (Though the opponent could still respond afterwards with a fast or burst spell, as always ... but the original spell will always be negated by either of these protective spells.) Note - I've been playing the game for about a year and a half, and I actually just learned something by thinking through this question.

TL;DR - The target of your spell was obliterated, so your spell no longer had a target and therefore fizzled.

Thanks for the detailed response! So by obliterating his own unit and replacing it with a new landmark, my opponent made it so the original target of my spell changed/original target didn't even exist anymore... I get it. In the theoretical case where something was transformed into a landmark (don't know if such a thing exists) it would work. Makes sense, I will have to treat obliterate a bit differently from now on.