Did our entering the Eye of the Universe trigger the universe's heat death? Obviously, the entire universe dies and is created anew, but throughout the game, ours is the only star in its final stages of life, yet the whole freakin cosmos dies in the last few minutes of gameplay (signified by all those lights going out in that trippy forest), so I'm thinking our interaction or mere observation of the Eye had to be the catalyst (though since we exist in a different quantum state entirely by that point, I suppose it's possible that the last few minutes of gameplay span billions of years in game time).
Ah, when you say "throughout the game, ours is the only star in its final stages of life", that's not true--go hang out with the guy at the campfire on Ember Twin, talk to him mid-way through the time loop, and then again and the end of the time loop, and they'll have some interesting stuff to point out for you!
Or if you don't feel like loading up the game, I'll tell you: all the other stars in the sky are also exploding as the 22 minutes pass!
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People who don't finish their sentences I'm a Castlevania superfan! Ask me anything!