The Spikey tool is a polygon tool that takes selected polygons and adds a vertex to the center of the polygon. It then repositions that vertex away from the original polygon based on the spike strength setting to create a spike. Modo creates spikes based on polygon face normals for each selected polygon.
You can access the Spikey tool by clicking the Polygon tab of the Modo Tools toolbar.
You can work with the Spikey tool interactively in the viewport, or you can adjust options in the tool's Properties panel and click Apply.