Albert Barnes Commentary Numbers 26:10

"and the earth opened its mouth, and swallowed them up together with Korah, when that company died; what time the fire devoured two hundred and fifty men, and they became a sign." — Numbers 26:10 (ASV)

Together with Korah - that is, they were engulfed at the same time that Korah perished, for Korah himself appears to have died among the two hundred and fifty incense offerers at the door of the tabernacle, not with Dathan and Abiram (compare Numbers 16:32 note).