Take Shadowmark Talon when you play Raven Guard and want a board-control army that is hard to shoot and hard to pin down. The honest comparison is with Vanguard Spearhead, which is also 2 DP and has the same core cover rule available to every Chapter.
What Shadowmark Talon adds is mobility and repositioning. Vanguard's suite leans defensive and sniper-flavoured; Shadowmark Talon gives you Feint and Thrust for Falling Back and still acting, Raptorial Vigilance for a reactive move, Into Darkness for a mass disengage, and Hunter's Instincts to come back early β plus Aethon Shaan making the disengage free once a round. It is the more restless version of the same idea, and it is chapter-locked in exchange.
At 2 DP it leaves 1 DP spare at 2,000 points; Subversion Assets is a natural partner, since both play the covert scoring game.
Sample gameplan: infiltrate a screen with Blackwing Shroud to seize the midfield and deny reserves turn one. Sit units where anything shooting from over 12" away gives you the Benefit of Cover, and snipe priority targets with Eliminators plus Stunning Fusillade. When the opponent finally commits into you, spend Raptorial Vigilance to dodge, or pull the unit out entirely with Into Darkness and bring it back early with Hunter's Instincts. Win on primary and secondary scoring, not on wiping the table.