1- We don’t exactly know how Dhul Fiqar looked like because there are different reports about it. In one hadith, the Imam (a) says it’s called Dhul Fiqar because any person it strikes it cuts them off from their family in this world and from heaven in the Hereafter. One description given to the sword is that it it’s not a straight sword—it has bulging parts and troughs, or that it has incisions in it. Some have said that it’s a two-headed sword, but there doesn’t seem to be any hadith evidence for that.
2- We understand from hadiths that this sword was kept with Um Salama in Medina and not taken to Karbala. One possible reason is that Allah had allowed for Imam Hussain (a) to be martyred, and had he taken the sword with him, his enemies might have been completely destroyed.