Grass turns to ash
Imam Baihaqui (May Allah shower His mercy on him) relates in Dalaail Un Nubuwwah:
Translation: Hadhrat Sufyan (May Allah be well pleased with him) narrates on the authority of his grandmother, who says: At the time of the martyrdom of Imam Hussain (May Allah be well pleased with him), I saw that the grass had turned to ash and I also saw that meat had become like fire. (Dalaail Un Nubuwwah, Hadith No. 2813)
This shows that for every person who had participated in the martyrdom of Hadhrat Imam Hussain (May Allah be well pleased with him), life had become hell and they would always be bewildered and anxious not knowing what punishment of Allah would be expressed from where. The fire was to show them that their place in hereafter was hell and this is only a taste of what was to come.