When the lamb opened the fifth seal, I saw under the altar the souls of those who had been slaughtered because of the word of God and the testimony they had given. They cried out in a loud voice, "Holy and true Sovereign, how long will it be before you judge and take revenge on those living on the earth who shed our blood?" Each of them was given a white robe. They were told to rest a little longer until the number of their fellow servants and their brothers was completed, who would be killed as they themselves had been: (Rev 6:9-11).
The souls under the altar are the souls of all the Christian martyrs killed during the Roman Empire (the Leopard beast) and the Holy Roman Empire (the Ram). These souls are crying to God for vengeance but they are told to wait for their fellow Christians martyrs who will soon join them. Even now, Christians are still being killed and persecuted under the false prophet. Islam Jihadist are slaying many Christians. The day of the Lord will come soon after the martyrdom of saints is complete.
The prayer of the saints beneath the altar brings the great day closer. That great and terrible day is described in the 6th seal and the 7th trumpet. The first four seals occur at once and then the 5th. The 6th and 7th seals are intertwined.