“At thy right hand there are pleasures for evermore” (Ps. 16: 11)
“Fear none of those things which thou shalt suffer: behold, the devil shall cast some of you into prison, that ye may be tried; and ye shall have tribulation ten days: be thou faithful unto death, and I will give thee a crown of life. He that hath an ear, let him hear what the Spirit saith unto the churches; He that overcometh shall not be hurt of the second death” (Rev. 2:10–11).
The church at Smyrna was standing at the edge of persecution. The Lord encourages them in that place by telling them not to fear what they are about to suffer. This implies two things. One is that they will in fact suffer, and the other is that in Christ they have no need to fear it. The specific persecution they faced was going to be stirred up by the devil. Some of them would be thrown into prison so that they might be tried or tested. The tribulation would last for ten days. We can assume that it was going to end in death for some of them, because the Lord promises the crown of life for those who were faithful to the point of death.
The one who has an ear should take heed and hear. This is what the Spirit is saying to the churches (plural). The second death is something to be feared, and those who are faithful through the first death need not fear it.
We do not have any extra-biblical record of a persecution there lasting for ten days, but there is no real reason we should. Because this persecution is said to be one coming from the devil, and a moment earlier they were told that there was a synagogue of Satan there, it is likely that this would be a persecution arising from unbelieving Jews.