The way I see it now, we have two conflicting descriptions of what the spell does, but neither would cause the death of Tarquin.

Quote Originally Posted by Darth V
Every living creature that directly shares your bloodline is dead. Every living creature that is directly related to any of those creatures is also dead. Anyone who could possibly make a claim to be part of your family is gone now.
Assuming "direct bloodline" means "-parents" and "-children", this would happen:

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The people in red (including all of the omitted Draketooth clan) were all killed, and caused the death of the people in pink (who are either members of branching family paths, or people who "married" into the main branch), and then it would stop. The families of Penelope and the other people tricked by the Draketooth would not be wiped "to the last cousin", only a direct line of ascendency/descendancy would be killed. That, of course, contradicts:

Quote Originally Posted by Guilty V
The Familicide spell I cast killed everyone of the black dragon's blood – and then killed everyone who shared blood with any of the dragon's blood.

[...]

I have scourged the Western Continent of humans whose only sin was falling in love with a mysterious scarlet-tressed stranger! And I have estinguished their own family lines as well! Countless parents, siblins, and children, dead – down to the last cousin.
This definition ignores the "direct" part, and says everyone "with the blood of the black dragon is killed". This means cousins and uncles/aunts of the ABD would be part of the first loop, and should be painted in red instead of pink. This would also mean most of the people in blue in the first chart are now in pink.

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However, this definition still wouldn't have killed Tarquin even if he had a child with Penelope. The second loop was targeted at Penelope, not the hypothetical child. Everyone who shared blood with her would be killed. And, as much as Tarquin would share blood with the kid, he'd still not share blood with her.