Damage that would be dealt by Excruciator can't be prevented.
A creature dealt lethal damage by Excruciator can be regenerated.
An effect may redirect Excruciator’s damage.
Damage prevention shields that would prevent this damage aren’t used up and they stick around for the next time something would deal damage.
Excruciator can deal damage to creatures with protection from red. (It can’t block creatures with protection from red, though.)
Replacement effects that don’t use the word “prevent” can replace Excruciator’s damage with something else. See Phytohydra and Szadek, Lord of Secrets, for example.