Let's talk Nightblade since the profession has garnered interest in other threads.
One of my goals in writing HARP Subterfuge was to avoid the "creep factor" (i.e. adding more and more skills and professions) that eventually unbalances a gaming system. In this book I propose an alternative way for players to create the type of thief or rogue they want to role play (e.g. assassin, burglar, spy, pickpocket, etc.), but even with this method there are still professions that can not be formed using the existing subterfuge archetypes. The Nightblade is one such profession.
For the Nightblade, I wanted to respect the heritage of the profession from Rolemaster but add new value and interest to it. Fundamentally, the Nightblade "...is a master of movement, distraction, disguise, and poison who uses magic to complement and enhance their covert operations." A magic-using spy, to be brief.
So I wondered: how did the profession come to be? Espionage is typically purview of politics—a tool of the nation-state to establish or retain dominance over its enemies. It seemed more likely that the Nightblade was created out of some need rather than morphing from some existing magical profession dabbling in subterfuge.
From this realization I wrote a hypothetical backstory to explain the origins of the profession. This story helped me shape some of the supporting material elsewhere in HARP Subterfuge (e.g. considerations for Stalking & Hiding, details for Disguise and Infiltration), helped expand Poisoning to a full-fledged chapter of its own, and guided the development of the Sphere of Shadows—a list of 29 spells based around the ideas of movement, distraction, disguise, and poison.
Seventeen of the spells in this Sphere are new. At the risk of revealing too much, the spell Contaminate Blade allows the caster to imbue her weapon with caustic magic that can do additional damage, and Dark Poisons allows the caster to create a variety of magically poisonous substances in order to incapacitate targets. Some other new spells are Dark Forms, Leaping, and Phantom Passing.
Overall, with this robust spell sphere and the supporting material, a player can actually create a Nightblade with a specific focus and mastery that fits their roleplaying vision.
Allow me to end this post with an excerpt from my Nightblade origin story.
He screamed for weeks in the depths of the Tower, as sphere after sphere of magic were infused into his blood. Thaumaturgy, Elementalism, Necromancy, Vivamancy, and even Ranger and Harper, were seeped slowly and painfully into his veins. When all was done and the Second recovered from the ordeal, the Grand Magus handed him a black-handled knife and said, "Go forth, our Blade of the Night. See your task done."