A sample of inherit from Decorator class. CalloutBorder wraps content with custom shape border, parameters of pointer are customizable. Sample is also attached.

CalloutBorder.cs

using System.Windows;
using System.Windows.Controls;
using System.Windows.Media;

namespace iburlakov
{
    public class CalloutBorder : Decorator
    {
        private const double DEFAULT_POINTER_HEIGTH = 20D;
        private const double DEFAULT_POINTER_WIDTH = 10D;

        public static readonly DependencyProperty BorderBrushProperty = DependencyProperty.Register("BorderBrush", typeof(Brush), typeof(CalloutBorder));
        public Brush BorderBrush
        {
            get { return this.GetValue(BorderBrushProperty) as Brush; }
            set { this.SetValue(BorderBrushProperty, value); }
        }

        public static readonly DependencyProperty BackgroundProperty = DependencyProperty.Register("Background", typeof(Brush), typeof(CalloutBorder));
        public Brush Background
        {
            get { return this.GetValue(BackgroundProperty) as Brush; }
            set { this.SetValue(BackgroundProperty, value); }
        }

        public static readonly DependencyProperty BorderThicknessProperty = DependencyProperty.Register("BorderThinckness", typeof(double), typeof(CalloutBorder));
        public double BorderThickness
        {
            get { return (double)this.GetValue(BorderThicknessProperty); }
            set { this.SetValue(BorderThicknessProperty, value); }
        }

        public static readonly DependencyProperty CornerRadiusProperty = DependencyProperty.Register("CornerRadius", typeof(double), typeof(CalloutBorder));
        public double CornerRadius
        {
            get { return (double)this.GetValue(CornerRadiusProperty); }
            set { this.SetValue(CornerRadiusProperty, value); }
        }

        public static readonly DependencyProperty PointerHeigthProperty = DependencyProperty.Register("PointerHeigth", typeof(double), typeof(CalloutBorder), new FrameworkPropertyMetadata(CalloutBorder.DEFAULT_POINTER_HEIGTH, FrameworkPropertyMetadataOptions.AffectsRender));
        public double PointerHeigth
        {
            get { return (double)this.GetValue(PointerHeigthProperty); }
            set { this.SetValue(PointerHeigthProperty, value); }
        }

        public static readonly DependencyProperty PointerWidthProperty = DependencyProperty.Register("PointerWidth", typeof(double), typeof(CalloutBorder), new FrameworkPropertyMetadata(CalloutBorder.DEFAULT_POINTER_WIDTH, FrameworkPropertyMetadataOptions.AffectsRender));
        public double PointerWidth
        {
            get { return (double)this.GetValue(PointerWidthProperty); }
            set { this.SetValue(PointerWidthProperty, value); }
        }

        public static readonly DependencyProperty PointerPositionProperty = DependencyProperty.Register("PointerPosition", typeof(double), typeof(CalloutBorder), new FrameworkPropertyMetadata(0D, FrameworkPropertyMetadataOptions.AffectsRender));
        public double PointerPosition
        {
            get { return (double)this.GetValue(PointerPositionProperty); }
            set { this.SetValue(PointerPositionProperty, value); }
        }

        protected override Size ArrangeOverride(Size arrangeSize)
        {
            if (this.Child != null)
            {
                var contentRect = new Rect(this.PointerWidth, 0, arrangeSize.Width - this.PointerWidth, arrangeSize.Height);

                var borderMove = -1 * this.BorderThickness / 2;
                contentRect.Inflate(borderMove, borderMove);

                this.Child.Arrange(contentRect);
            }

            return arrangeSize;
        }

        protected override void OnRender(DrawingContext drawingContext)
        {
            var borderRect = new Rect(0, 0, this.RenderSize.Width, this.RenderSize.Height);
            var borderMove = -1 * this.BorderThickness / 2;
            borderRect.Inflate(borderMove, borderMove);

            // draw border with pointer
            var geometry = new StreamGeometry();
            using (var context = geometry.Open())
            {
                // validate PointerHeigth
                double pointerHeigth;
                if (this.PointerHeigth < CalloutBorder.DEFAULT_POINTER_HEIGTH || this.PointerHeigth >= borderRect.Height)
                {
                    pointerHeigth = CalloutBorder.DEFAULT_POINTER_HEIGTH;
                }
                else
                {
                    pointerHeigth = this.PointerHeigth;
                }

                // validate PointerPosition
                var minPointerMargin = 2;
                var minPointerPosition = borderRect.Y + pointerHeigth / 2 + this.CornerRadius + minPointerMargin;
                var maxPointerPosition = borderRect.Height - pointerHeigth / 2 - this.CornerRadius - minPointerMargin;

                double pointerPosition = System.Math.Abs(this.PointerPosition);
                var positionFromBottom = this.PointerPosition < 0;
                if (pointerPosition > maxPointerPosition)
                {
                    pointerPosition = positionFromBottom ? minPointerPosition : maxPointerPosition;
                }
                else if (pointerPosition < minPointerPosition)
                {
                    pointerPosition = positionFromBottom ? maxPointerPosition : minPointerPosition;
                }

                // top-left corner
                context.BeginFigure(new Point(borderRect.X + this.PointerWidth + this.CornerRadius, borderRect.Y), true, true);
                context.ArcTo(new Point(borderRect.X + this.PointerWidth, borderRect.Y + this.CornerRadius), new Size(this.CornerRadius, this.CornerRadius), 0, false, SweepDirection.Counterclockwise, true, false);

                if (positionFromBottom)
                {
                    // line to pointer
                    context.LineTo(new Point(borderRect.X + this.PointerWidth, borderRect.Height - pointerPosition - pointerHeigth / 2), true, true);

                    // draw pointer
                    context.LineTo(new Point(borderRect.X, borderRect.Height - pointerPosition), true, true);
                    context.LineTo(new Point(borderRect.X + this.PointerWidth, borderRect.Height - pointerPosition + pointerHeigth / 2), true, true);
                }
                else
                {
                    // line to pointer
                    context.LineTo(new Point(borderRect.X + this.PointerWidth, borderRect.Y + pointerPosition - pointerHeigth / 2), true, true);

                    // draw pointer
                    context.LineTo(new Point(borderRect.X, borderRect.Y + pointerPosition), true, true);
                    context.LineTo(new Point(borderRect.X + this.PointerWidth, borderRect.Y + pointerPosition + pointerHeigth / 2), true, true);
                }

                // line to bottom-left corner
                context.LineTo(new Point(borderRect.X + this.PointerWidth, borderRect.Height - this.CornerRadius), true, true);

                // bottom-left corner
                context.ArcTo(new Point(borderRect.X + this.PointerWidth + this.CornerRadius, borderRect.Height), new Size(this.CornerRadius, this.CornerRadius), 0, false, SweepDirection.Counterclockwise, true, false);

                // bottom line
                context.LineTo(new Point(borderRect.Width - this.CornerRadius, borderRect.Height), true, false);

                // bottom-rigth corner
                context.ArcTo(new Point(borderRect.Width, borderRect.Height - this.CornerRadius), new Size(this.CornerRadius, this.CornerRadius), 0, false, SweepDirection.Counterclockwise, true, false);

                // rigth line
                context.LineTo(new Point(borderRect.Width, borderRect.Y + this.CornerRadius), true, false);

                // top-left corner
                context.ArcTo(new Point(borderRect.Width - this.CornerRadius, borderRect.Y), new Size(this.CornerRadius, this.CornerRadius), 0, false, SweepDirection.Counterclockwise, true, false);
            }

            drawingContext.DrawGeometry(this.Background, new Pen(this.BorderBrush, this.BorderThickness), geometry);
        }
    }
}

MainWindow.xaml

<Window x:Class="Test.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" SizeToContent="WidthAndHeight"
        xmlns:c="clr-namespace:iburlakov">

    <Grid Width="365">
        <Grid.RowDefinitions>
            <RowDefinition Height="200" />
            <RowDefinition Height="20" />
            <RowDefinition Height="auto" />
            <RowDefinition Height="auto" />
            <RowDefinition Height="auto" />
        </Grid.RowDefinitions>

        <c:CalloutBorder x:Name="callout" Grid.Row="0" BorderBrush="Green" BorderThickness="1" CornerRadius="5" Width="175" Margin="0 20"
                         PointerPosition="80" PointerHeigth="20" PointerWidth="10">
            <TextBlock VerticalAlignment="Center" HorizontalAlignment="Center">
                line 1
                <LineBreak />
                line 2
                <LineBreak />
                line 3
                <LineBreak />
                line 4
            </TextBlock>
        </c:CalloutBorder>
 
        <StackPanel Grid.Row="2" VerticalAlignment="Center" Orientation="Horizontal">
            <Label Content="Pointer Position" Width="100" />
            <TextBox Width="50" Text="{Binding PointerPosition, ElementName=callout}" />
            <Slider Value="{Binding PointerPosition, ElementName=callout}" Minimum="0" Maximum="200" Width="200" />
        </StackPanel>

        <StackPanel Grid.Row="3" VerticalAlignment="Center" Orientation="Horizontal">
            <Label Content="Pointer Heigth" Width="100" />
            <TextBox Width="50" Text="{Binding PointerHeigth, ElementName=callout}" />
            <Slider Value="{Binding PointerHeigth, ElementName=callout}" Minimum="0" Maximum="100" Width="200" />
        </StackPanel>
        
        <StackPanel Grid.Row="4" VerticalAlignment="Center" Orientation="Horizontal">
            <Label Content="Pointer Width" Width="100" />
            <TextBox Width="50" Text="{Binding PointerWidth, ElementName=callout}"/>
            <Slider Value="{Binding PointerWidth, ElementName=callout}" Minimum="0" Maximum="100" Width="200" />
        </StackPanel>
   </Grid>
</Window>