How large does an object have to be to cause global extinction?

I've been trying to explain the origins of a fantasy world I've been constructing. So far, I have the idea of a Promethean (advanced race of aliens) Penal ship crashing (probably from an internal uprising) into the planet.

I need two things to still remain on the planet:

1. Habitable atmosphere so the damage caused by the crash should only be enough to cause an extinction event on the surface.
2. The ability for some ocean and cave creatures to survive.

Now what would be the size (as in what scope do I have to work with) of the ship to cause an extinction upon impact but not make the world uninhabitable.

Note: Due to the nature of the ship, when the ship begins descending to the surface the crew and prisoners would be directed to stasis pods that would allow some to survive the crash. On top of this, the ships AI would only wake them once the atmosphere is safe again (handwave how they can breath the specific atmosphere for now).

Specifics: ship is 2 kilometers in diameter (weight will be added soon) and speed would be roughly 6-8000 kilometers per hour.

posted over 6 years ago

Celestial Dragon Emperor‭

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