Walk on Water

Eyal is an agent in Mossad, the Israeli security service. He is a hitman who targets enemies of Israel. His wife has recently committed suicide, and the agency decides that he needs to take on a less challenging assignment: to find an aging Nazi war criminal and get him "before God does".
In order to track down the old man, Eyal poses as a tour guide and befriends the Nazi's adult grandchildren, Axel and Pia. Pia lives on a kibbutz, an Israeli commune. Her brother Axel visits her in order to convince Pia to return to Germany for their father's seventieth birthday. It is later revealed that Pia's estrangement with her parents began when she discovered that they were hiding her grandfather. She shares this information with Axel.
Although he has a job to perform, Eyal truly befriends Axel and Pia. Axel and Pia are decent people who demonstrate that most Germans have gotten beyond the hatred that led to the Holocaust. They spend time together and Eyal enjoys himself, even if he would not openly admit so. His friendship with Axel allows him to display some humanity, letting down his tough-guy machismo. Eyal and Axel even take a mud bath by the Dead Sea, showering off together in the nude afterward.
When the three are at dinner one night in a Tel Aviv restaurant, Axel speaks privately to the Palestinian waiter, Rafik, and finds out where the best club in town is. Later that evening, Axel, Pia, and Eyal arrive at the club. Eyal is shocked to discover that it is a gay club. He sees Axel dancing with Rafik and is taken aback.
Eyal is initially disgusted and disappointed to discover that Axel is gay. He asks to be removed from the assignment, not attempting to hide his homophobia as the reason. His boss, Menachem, insists that Eyal finish the mission. Eyal visits Germany and comes to realize that Axel's orientation is unimportant. During the visit, Eyal defends a group of Axel's transsexual friends from attackers and, in doing so, reveals that he is fluent in German. He tells Axel that his parents were German, leaving out that most of the Jews in his mother's region of Germany had been killed by Axel's grandfather. Later during the visit, Eyal runs into Menachem, whose family was also killed by the grandfather's Nazi activities.
Axel invites Eyal to his father's birthday party. The guests are uncomfortable about Eyal's nationality and religion, but still polite. After the cake is brought out, Axel's parents surprise the guests by bringing out Axel's aged grandfather. Axel angrily confronts his mother and goes to Eyal's room, only to find a folder full of information on Axel's family. Meanwhile, Eyal meets with Menachem and tells him that they can easily take the grandfather and bring him to Israel to be tried for his war crimes. Menachem reveals that they are the only two on this mission, and the aim is to kill the grandfather. Eyal is clearly conflicted, but takes the case of poisons that Menachem gives him.
Eyal arrives at Axel's house and enters the grandfather's room, unbeknownst to all but Axel. Axel sneaks up behind Eyal and watches as he fills a syringe with poison, doing nothing to intervene. Ultimately, though, Eyal is unable to fulfill the task. He leaves, and Axel tenderly caresses his grandfather's face before turning off his oxygen tank, killing him. He goes to Eyal's room, where Eyal tells him that the suicide note his wife wrote told him that he kills everything that comes near him. Eyal says that he doesn't want to kill anymore and breaks down in Axel's arms.
The story jumps ahead 2 years. Eyal and Pia are married with a child named Tom and living on the Kibbutz. Eyal and Axel remain good friends.

Question: Who is the special guest in the birthday party?
Answer: Axel's Grandfather.