public class Mention
    {
        [BsonRepresentation(BsonType.ObjectId)]
        public string Id { get; set; }
       
        public RelatedData Related { get; set; }
    }

    public class RelatedData
    {
        public List<int> Players { get; set; }
        public List<int> Clubs { get; set; }
    }

_context.For<Mention>().AsQueryable()
               .Where(...)
               .SelectMany(x => x.Related.Players)
               .ToList()

{  "$match" : {  ....  }  },
{  "$unwind" : "$Players" } <--- must be "$Related.Players"