In the Mishna Berurah on the end of Siman 306 (page 200 of Vol. 3 in the classic editions) he writes:
כתב כנה"ג דללכת חוץ לתחום בשבת לנקום נקמת אביו מרוצחים מותר ותמה עליו המג"א וש"א דהא לא עדיף משאר צורך מצוה דאסור לילך חוץ לתחום עבורה:
In the Sefer Knesset Hagedola - כְּנֶסֶת הגְּדוֹלָה - it is written one is permitted to leave the (2000 Amot) Shabbat boundary in order to avenge one's father revenge from murderers.
The Magen Avraham and Sha'agat Arye wonder about this since it's no different from any other Mitzvah that one can't leave the Shabbat boundary to fulfill.
What exactly is this לנקום נקמת אביו מרוצחים - avenging one's father revenge from murderers - that we are discussing?
If it's talking about a Biblical Honour Killing (as in גּוֹאֵל הַדָּם) then why only a father? All close relatives should have the same status. (Rambam says וְכָל הָרָאוּי לִירֻשָּׁה הוּא גּוֹאֵל הַדָּם - anybody who qualifies for the inheritance is a גּוֹאֵל הַדָּם).
What revenge is the Knesset Hagedola referring to?