This is a simple, tangy sauce that's very good with crab, lobster, shrimp, or any other sweet-fleshed shellfish.
If you're good in the kitchen and like adjusting recipes to match your individual tastes, here's the meta-recipe: start with about as much ketchup as you want sauce. Add horseradish until the ketchup looks like it has horseradish in it - that is, until lighter shreds are visible. Mix in lemon juice until the desired level of tartness is achieved.
For those less confident types (like me), here's a recipe.
Mix well, enjoy.