Here are the two I have so far:
Play Magnus Achievements

The 'secret' achievements that I have yet to unlock are:
Horsemen of the Apocalypse
King of the Impossible
Lightning Strike
Murphy's Law
The Art of Blackmailing

This week's achevment is a Tribute to Nakamura. In order to get Men of Good Faith you just need to promote all of your pawns into bishops.

For "King of the Impossible," I've tried checkmating with King Move (i.e. Castle) and moving the king to every square on the board, and niether has worked. Any ideas?

Just tried a discovered King checkmate against Magnus 6 1/2, but did not work. Could it have to do with surviving consecutive checks?

They added 8 new achievements this week:
As Keen as Mustard (250) - secret
Cannon Fodder (0) - secret
Hiding in the Castle (0) - Used Castling Move to checkmate [I've tried this one on Magnus 5 and 6 but it didn't work]
Master of the 10th (5) - win 5 in a row against Magnus 10
Master of the 11th (5) - win 5 in a row against Magnus 11
Pitchforks and Torches (1) - Checkmate with only King and Pawn [I tried this on Magnus 5 and it didn't work]
Thirsty for More (750) - Secret
Waving the White Flag (10) - Secret [but I imagine that it has to do with resigning]
Has anyone else tried Pitchforks and Torches or Hiding in the Castle and gotten them to work?
I have also tried both pitchfork and hiding in castle. Didn't work. Has reported it as a bug.
Tried 5 and 6 year

@mingbrothers My strategy was to keep as much material on the board as possible. It took over 15 tries against Magnus 5 but I was able to get him with just moving my two knights.
Note that it is impossible to checkmate him with just the two knights, you have to use his own material against him. Thus avoiding captures, esp around Magnus's king is a good idea.

@mingbrothers we don't know what "Thirsty for More" is yet (or if it is even functioning yet), but with 750 required it likely has to do with captures or game wins.
Murphy's Law requires that you get checkmated by Magnus 5.

We think Keen as Mustard, Waving the White Flag, and Cannon Fodder were prematurely published and as a result are not functioning correctly.
Still trying to figure out King of the Impossible, see previous posts for discussion on it.
Lightning Strike is beating Magnus in three moves or less. Basically farm Magnus 5 and hope that he lets you fool's mate him.
Hi all,
The play Magnus app recently added new weekly secret achievements. Has anyone figured out what the secret achievements are?
All known completed achievements: