Use the straight line and bezier curve tool to draw a spiky ended shape.
Add more spikes to it using the straight line tool.
Alternatively create a triangle and then add more nodes…
…to move into place with the edit paths by nodes tool.
Add a few more cracks before grouping the shapes (CTRL+G).
Duplicate the group and offset the position slightly (matching your scene’s light setup)…
… move the end nodes to line up with our initial shape group…
… and choose a lighter fill colour and set the alpha to ~50% (adjust to your window colour).
Place the lighter shape below the darker group.
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