Part of the Blackmar-Diemer Gambit: see the full opening guide
The Halosar Trap comes out of the Ryder Gambit, the double-pawn sacrifice version of the Blackmar-Diemer Gambit, and it is named after Hermann Halosar, who fell into it against Emil Josef Diemer at Baden-Baden 1934. White gives up two pawns, then the queen itself. If Black takes everything, a quiet knight hop to b5 turns into checkmate on c7.
White sacrifices the d4 and f3 pawns for fast development and castles long. When Black attacks the queen with 7...Bg4, White ignores it: 8.Nb5! threatens Nxc7#, and if Black grabs the queen with 8...Bxf3, then 9.Nxc7# is mate on the spot.
After 1.d4 d5 2.e4 dxe4 3.Nc3 Nf6 4.f3 exf3 5.Qxf3 (the Ryder Gambit), Black is two pawns up once the queen takes on d4. But look at the development count: after 6.Be3 Qb4 7.O-O-O, every White piece is working and the d1 rook owns the open d-file, while Black has moved only the queen and one knight. 7...Bg4? attacks the queen and expects to win more time. Instead 8.Nb5! ignores the attack completely: the knight threatens 9.Nxc7#, mate, because the e7 and f7 pawns box in the king and the d1 rook covers d7 and d8. Black cannot remove the knight either, since 8...Qxb5 runs into 9.Bxb5+, saving the white queen with check. So when Black plays the greedy 8...Bxf3??, taking the queen, White answers 9.Nxc7# and the game is over with a full queen still sitting in Black's pocket.
8… Bxf3 — 8...Bxf3?? (ply 16) is the losing move: Black wins the queen and gets mated at once by 9.Nxc7#. The bait was laid one move earlier with 7...Bg4?, which attacks the queen but ignores the real story, the loose c7 square and White's rook on the open d-file. After 8.Nb5 the only way to fight on was 8...Na6, covering c7 and declining everything.
As Black, respect the development gap in the Ryder Gambit: taking on d4 is already playing with fire, and 7...Bg4? is the move that walks in. If you have gone this far, do not touch the queen. Cover c7 with 8...Na6; after 9.Qxb7 Qe4 10.Qxa6 Qxe3+ 11.Kb1 Qc5 12.Nf3 Black has avoided instant loss, though White keeps real pressure for the material. In the original game Halosar tried 9...Rc8 and lost after 10.Qxa6, when the knight simply falls.
Objectively no: White gives up two pawns, and theory says Black can defend and keep the material. Even Blackmar-Diemer specialists admit as much. It is a practical weapon: one greedy or careless move and Black gets mated, which is exactly what this trap shows.
Because 8...Qxb5 is answered by 9.Bxb5+, check. The bishop recaptures with tempo, the white queen on f3 escapes the attack from the g4 bishop, and White remains massively ahead in development. That little zwischenzug is what makes 8.Nb5 safe.
A trap only works if your opponent makes the mistake — strong players sidestep these, which is why each page also shows how to avoid it. Every line here is checked legal with the same engine the board runs, and every checkmate is verified.