first, you may have omitted 2 letters, please check, the sense depends on every. It must be Совершившиеся
If it is Совершившиеся then the meaning is ‘those who finished their action(s)’ active voice, past tense, plural. This is descriptive word formed out of the verb ‘to happen, to finish’ in the most general meaning.
The exact sense depends hugely on the context, if you could give the whole phrase I could help.
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Happy to be of service, you’re welcome 🙂