וַתִּפְתַּ֨ח הָאָ֜רֶץ אֶת־פִּ֗יהָ וַתִּבְלַ֥ע אֹתָ֛ם וְאֶת־קֹ֖רַח בְּמ֣וֹת הָעֵדָ֑ה בַּאֲכֹ֣ל הָאֵ֗שׁ אֵ֣ת חֲמִשִּׁ֤ים וּמָאתַ֙יִם֙ אִ֔ישׁ וַיִּהְי֖וּ לְנֵֽס׃
地開了口，把他們和可拉一同吞下去；那一黨的人死的時候，火燒滅了二百五十個人，他們就成了警戒的記號。
Rashi
ויהיו לנס AND THEY BECAME A נס — a sign and a reminder in order that no stranger should bring himself near to raise further dispute concerning the priesthood (cf. Numbers 17:5).
Rabbeinu Bahya
ותבלע אותם ואת קרח במות העדה באכול האש, “it (the earth) swallowed them and Korach when the congregation died when the fire consumed them;” the verse introduces the name Korach between the words “it swallowed” and the words “when the fire consumed,” in order to teach us that Korach was both swallowed by the earth and burned by the fire. (I already mentioned this in Numbers 16,30).
Siftei Chakhamim
A symbol and a reminder. That is to say that the term נס ["banner"] has the connotation of something high [which is visible to all], similar to כנס על הגבעה ["like a banner on top of the hill"] (Yeshayahu 30:17), and כנשוא נס הרים ["a banner raised over the mountains"] (Yeshayahu 18:3).