Undertaker's fresh scars have been making me wonder about the timeline. But also whether the whole necromancy thing may have something to do with the Grim reapers.
In chapter 149, Othello states that Undertaker "attempted to escape from the organization around seventy years ago and half destroyed headquarters in the process". "Attempted to" is very pointed wording. Why not just say he escaped outright if he was successful? Othello seems to be implying that Undertaker failed at that time, but he gives no indication of when he actually escaped for good, only that he is a "fugitive of legend". Back in chapter 60, Undertaker himself says that it had been "at least half a century" since he was called a Grim reaper. Assuming this isn't a mistake on Yana's part or a mistranslation by the EN team, and I'm not misunderstanding it, Undertaker may have attempted to escape 70 years ago and only succeeded 20 years after the fact (assuming that there were no other attempts in those 20 years).
Then where does this put him in Modri's flashback? Judging from their clothes at the ball, Modri's flashback may take place around the end of the 18th century/beginning of the 19th century. I couldn't find an actual year for when his contract with Anna takes place, so the only thing I can do is guess. After joining the exorcists, Modri doesn't seem to get much older during his time with them before ultimately dying.
Then, what stage of his life is Undertaker in? Not only are his scars very much fresh, but he is also wearing a suit that looks a lot like the Reaper uniform. The only thing that makes sense to me is that Undertaker found Modri right after the attempt Othello describes. (I'm guessing that is when Undertaker got his scars because I doubt he could've gone unscathed when destroying heartquarters. He also may have been tortured for the attempt.) But then again, if he did pick Modri up right after his unsuccessful attempt, why would the Reapers have let him go so easily? There are several instances of Grell complaining how understaffed the Reapers are; did they think the wounds inflicted upon him would be enough to deter him from attempting to escape a second time? I did think about him possibly picking Modri up before his attempt 70 years ago, but I cannot explain why he'd have his scars then (as he didn't seem to have them when he was still a model Reaper). It especially doesn't make sense that he found Modri right after his successful escape 50 years ago, as Modri would be much older then.
But also, what is the point of his experimentation? We don't know when he got involved with the Phantomhives exactly, so it begs the question whether he started out doing it for them or for another reason. If he started experimenting either after attempting to or successfully escaping the Grim reapers, could the experiments have something to do with the reason he left/wanted to leave?