History - villain is a TAG but can be semi LAG at times, depending on the table. The type that never makes any huge ridiculous bluffs, imo. He's 3 betted me the last 4 hands I've opened pre flop in row and all 4 hands went check check flop and I led out turn and he folded on all 4 occasions. We're sitting with approx 15k deep effective.

Hand - I open $100 with Ac Ad UTG, folds around to villain on the button who makes it $300, comes back to me and I make it $900, villain calls. Flops 9c 8c 2d. However before the flop comes I lead out $1200 blind, V calls. Turn is 4h, I lead $1600, V tank calls. River is a 5h. I bet $3800 into $7400 and villain shoves for another $6000 on top.... Hero???

Why would you ever dark lead flop?

Fold. The dark bet usually is a combo of AK-Aa-qq with the occasional spazz hand. When we do this villain often due to the irregularity of the dark bet often calls with sometimes ATC to see what you will do on the turn, when we bet again and villain calls he now knows you probably don't have AK but just in case he still doesn't know we definitely know when he shoves over the top of you on the river. Any 1 pair hands he has he will just call you down with. If he's the type of player to not 5 bet kings because he doesn't want to play for stacks and or see a safe flop this same kind of player hardly ends up deciding on the river after we 4 bet and barrel off to now ship in over the top of us on the river. Vs this player type I think it's a bet fold. I don't like the large sized cbet we made either. Don't let the tank call on turn fool you into thinking he was on some massive draw, when that is the case a decent player will usually want to call faster so that we slow down on the river. The tank call often is used for deception on draw heavy boards in hopes that the river brings a blank and we heros level ourselves into thinking villain missed everything.

bet/fold river, he's played pretty passively vs you in particular, partly because he's giving you a ton of credit or he is just trying to hit a flop to stack your perceived tight range