This is the active past participle случи́вшееся from the verb случиться to happen, which declines like any neuter adjective. Here, it's in the prepositional singular after o and means about what happened.
[Literally: about the occurred].
This is the past participle случи́вшееся from the verb случиться to happen, which declines like any neuter adjective. Here, it's in the prepositional singular after o and means about what happened.
[Literally: about the occurred].
This is the active past participle случи́вшееся from the verb случиться to happen, which declines like any neuter adjective. Here, it's in the prepositional singular after o and means about what happened.
[Literally: about the occurred].