public abstract class AreaBase : BusinessEntity
{
    public virtual string FirstName { get; set; }
    public virtual string LastName { get; set; }      
}

public class OArea : AreaBase
{
    public virtual Regions Region { get; set; }
}

public class Regions : List<RegionsArea>
{
}

public class RegionsArea : AreaBase
{
    public virtual Country Country { get; set; }
}

CREATE TABLE [dbo].[tArea]
(
    [Id] [dbo].[ID] IDENTITY(1, 1) NOT NULL PRIMARY KEY,
    [FirstName] varchar(50) NOT NULL,
    [LastName] varchar(50) NOT NULL,
    [ParentId] [dbo].[ID] NULL,
    CONSTRAINT [FK_tArea_tArea] FOREIGN KEY ([ParentId]) REFERENCES [tArea]([Id])
)

<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping assembly="Entities" namespace="Entities.Profiles" xmlns="urn:nhibernate-mapping-2.2">
  <class name="AreaBase" table="tArea">
    <id name="Id">
      <column name="Id" sql-type="bigint"/>
      <generator class="native"/>
    </id>
    <property name="FirstName" column="FirstName"/>
    <property name="LastName" column="LastName"/>
  </class>
</hibernate-mapping>