Nice concept and fits the theme of Darkness and Rogues. You could consider renaming the upped version Assassin, but the current name is cool too.
A few suggestions:
1) Put a comma after Cloaked (If cloaked, invisibility...) or some people could get confused and understand: "If cloaked invisibility (which could be the invisibility given by cloak) doesn't wear off..." and think you've forgotten to complete the sentence. Just a thought
2) More than gaining poison when there is a dagger in play, it would be cooler (IMHO) theme-wise if it caused ALL the daggers and stilettos (which are the typical rogues' and assassins' weapons) to become poisoned.
Nice card overall