Some ideas to tie the various skill lines of Nightblade together:
Cornerstone ability: Mark Target - Nightblade abilities in other skill-lines used against targets under the effect of Mark Target may be augmented.
Shadow - Shadow Cloak - Born From Shadow: Increases your damage done to Monsters by 10%, increased to 20% for instant attacks against Marked enemies. Attacking a Marked enemy from the Flank will refresh the duration. (snipe is fine, not sure what to call abilities that only do non-orange damage).
Shadow - Summon Shade - The Shade prioritizes Marked enemies, and applies Minor Brittle to them for 5s. (Solo Debuff access)
Siphoning - Strife - Increases the damage of this ability by 30% against Marked targets instead of healing you. (Never played Magblade, so not sure if this is too OP?)
Siphoning - Cripple - Immobilizes Marked targets. (PvP utility)