calamities from Heaven, as floods, famines, pestilence, etc., misery
烖 centers on large-scale disasters and suffering, especially those seen as coming from natural or heavenly forces: floods, famines, pestilence, and the misery they bring.
烖 is an older form of 災, combining 火 (fire) with a phonetic element derived from 才 or 哉. The 土 and 戈 components likely represent a stylized phonetic marker, while 火 indicates the destructive nature of the disaster.
Picture a scene where fire (火) breaks out from the ground (土) amid weapons (戈), bringing calamity and misery. The combination suggests a heaven-sent disaster.
For サイ, imagine a great calamity striking with a loud sigh: sigh -> サイ, as people lament the misery brought by floods and fire.