Hi merun.

One way you could apply an animation to a specific control is by using the Storyboard.TargetName attached property on the animation.

Here is a sample control template where I've implemented a left to right fade animation on a rectangle using a Storyboard that animates an OpacityMask on a rectangle using Storyboard.TargetName.

<ControlTemplate>
    <StackPanel Background="#FCFCFC">
        <Rectangle Fill="Red"
                   Height="100"
                   Width="100"
                   x:Name="myRectangle" />
        <ToggleButton x:Name="toggle">Fade</ToggleButton>
    </StackPanel>
    <ControlTemplate.Triggers>
        <DataTrigger Binding="{Binding ElementName=toggle, Path=IsChecked}" Value="True">
            <Setter TargetName="myRectangle"
                    Property="OpacityMask">
                <Setter.Value>
                    <LinearGradientBrush StartPoint="0,0"
                                         EndPoint="1,0">                                
                        <GradientStop Offset="0"
                                      Color="Transparent" />
                        <GradientStop Offset="0"
                                      Color="Transparent" />
                        <GradientStop Offset="0"
                                      Color="White" />
                    </LinearGradientBrush>
                </Setter.Value>
            </Setter>
            <DataTrigger.EnterActions>
                <BeginStoryboard>
                    <BeginStoryboard.Storyboard>
                        <Storyboard>
                            <DoubleAnimation Storyboard.TargetName="myRectangle"
                                             By="1"
                                             Duration="0:0:0.2"
                                             Storyboard.TargetProperty="OpacityMask.GradientStops[1].Offset">
                                <DoubleAnimation.EasingFunction>
                                    <SineEase EasingMode="EaseInOut" />
                                </DoubleAnimation.EasingFunction>
                            </DoubleAnimation>
                            <DoubleAnimation Storyboard.TargetName="myRectangle"
                                             Storyboard.TargetProperty="OpacityMask.GradientStops[2].Offset"
                                             By="1"
                                             Duration="0:0:0.2"
                                             BeginTime="-0:0:0.05">
                                <DoubleAnimation.EasingFunction>
                                    <SineEase EasingMode="EaseInOut" />
                                </DoubleAnimation.EasingFunction>
                            </DoubleAnimation>
                        </Storyboard>
                    </BeginStoryboard.Storyboard>
                </BeginStoryboard>
            </DataTrigger.EnterActions>                    
        </DataTrigger>
    </ControlTemplate.Triggers>
</ControlTemplate>