create table Entities(
    EntityId bigint not null identity(1, 1),
    Name nvarchar(64) not null,
    ParentEntityId bigint null
)
alter table Entities add constraint PK primary key (EntityId)
alter table Entities add constraint FK foreign key (ParentEntityId) references Entities(EntityId)

public class Entity
{
    [Required]
    public virtual long EntityId { get; set; }

    [Required]
    public virtual string Name { get; set; }

    public virtual long? ParentEntityId { get; set; }

    public virtual Entity ParentEntity { get; set; }
}

modelBuilder.Entity<Entity>()
            .HasOptional(e => e.ParentEntity)
            .WithMany()
            .HasForeignKey(e => e.ParentEntityId );