Good Shen guide written by a good Shen player. I tend to use this as a base build and modify it to my play style.

To answer your question, Captainsaki, magic pen marks are a primary. Armor is a primary for seals, not marks, and I'd rather prefer to avoid secondary runes when possible. Also, I tend to play an aggressive Shen that can still kill enemies and the magic pen helps me accomplish this.

Can you explain to me why Shen would get the magic pen runes instead of armor?

This is a well thought out guide. I always notice that Leviathan is extremely underrated. As a tank, you should be around almost every fight racking up a lot of assists and maybe a few kills. If you can get it full, that 15% is very nice. You have my +1.