Halloween Gambit

The Halloween Gambit (also known as the Müller–Schulze Gambit or Leipzig Gambit) is an aggressive chess opening gambit in which White sacrifices a knight early on for a single pawn. The opening is an offshoot of the normally staid Four Knights Game and is defined by the moves:
The theoretician Oskar Cordel reported in 1888 that Leipzig club players used the opening to dangerous effect, but he did not believe it was . Their name for it, "Gambit Müller und Schulze", was not after any players by those names, but rather a jocular German equivalent of "Smith and Jones", or, "Tom, Dick, and Harry". The modern name "Halloween Gambit" was given by the German player Steffen Jakob, who explained that "Many players are shocked, the way they would be frightened by a Halloween mask, when they are mentally prepared for a boring Four Knight's, and then they are faced with Nxe5."
White's objective is to seize the with pawns and drive back Black's knights. After 4... Nxe5, White usually plays 5. d4 (5.f4 does nothing for ), after which Black can retreat the attacked knight to either g6 or c6.
5...Ng6 retreat.
When Black retreats 5...Ng6, White chases the f6-knight with 6.e5. Then after 6...Ng8 7.Bc4, former world champion Max Euwe recommended 7...d5 8.Bxd5 c6, contending in volume 11 of his opening series that Black has a decisive advantage.
Instead of holding on to the extra piece with the usual 6...Ng8, a more logical continuation according to Eric Schiller is 6...Bb4, giving Black the better game after 7.exf6 Qxf6 with a lead in development and pressure in the center. 
5...Nc6 retreat.
When Black retreats 5...Nc6, White chases the knight again with 6.d5. Then Black has 6...Ne5 (the Main line), or 6...Bb4 (Pinski's move).
6...Ne5.
After 6...Ne5, White chases again with 7.f4. Then after 7...Ng6 the game usually continues 8.e5 Ng8 9.d6 cxd6 10.exd6 (see diagram). In this case White's attack is very dangerous, and likely to prevail in practical play, with the threat of Nb5–c7 difficult to defend satisfactorily. Precise defense may theoretically preserve Black's advantage but White has achieved their opening objectives.
Pinski's 6...Bb4.
GM Larry Kaufman wrote in 2004 that the Müller–Schulze Gambit is by 4...Nxe5 5.d4 Nc6 6.d5 Bb4 7.dxc6 Nxe4 8.Qd4 Qe7, which he attributes to the Polish IM Jan Pinski. In 2003 Pinski analyzed 9.Qxg7 Nxc3+ 10.Be3 Nd5+ 11.c3 Rf8 12.cxb4 Nxe3 13.fxe3 Qxb4+, concluding "Black is very close to winning". White may also continue with the less risky 9.Be3.
Halloween Gambit with colors reversed.
A similar gambit can be tried by Black: after 4.g3, Black can play 4...Nxe4 This line is arguably sounder than its White counterpart because White's 4.g3 has weakened the f3-square. Moreover, White cannot play the line recommended by Kaufman with colors reversed, because 5.Nxe4 d5 6.Nc3 d4 7.Bb5 dxc3 8.Nxe5? Qd5 9.Qe2? loses to 9...Qxh1+. However, with the pawn on g3, Nh4 is possible and it should be easier to castle.
Illustrative games.
The following speed chess games (played by Brause, a chess computer) show what can befall an unprepared player of the black pieces:
The next game, played in a Halloween Gambit thematic tournament, won the prize for the most spectacular game won by White.