How was I murdered by Nyan Cat?
Solution 1:
One of the enemies which you would have been fighting near the solar pillar would have been a Selenian.
They have a special ability which allows them to reflect, among other things,
- Sword beams, such as those from the Terra Blade
or of course the Meowmere.
You were unlucky enough to be hit by your own projectile. As the Meowmere has the highest base damage of any melee weapon, the reflected projectile can deal over 300 damage. Since you would have been on lower health due to all the enemies of the lunar events, this attack would have easily 1-shot you.