en_tn_condensed/rev/06/09.md

the fifth seal

"the next seal" or "seal number five" (See: rc://en/ta/man/translate/translate-ordinal)

under the altar

This may have been "at the base of the altar."

those who had been killed

This can be translated with an active verb. AT "those whom others had killed" (See: rc://en/ta/man/translate/figs-activepassive)

because of the word of God and the testimony which they held

Here "Word of God" is a metonym for the message from God and "held" is a metaphor. Possible meanings are 1) holding the testimony refers to believing God's word and testimony. Alternate translation: "because of the teachings of scripture and what they taught about Jesus Christ" or "because they believed the word of God, which is his testimony" or 2) holding the testimony refers to testifying about the word of God. Alternate translation: "because they testified about the word of God" (See: rc://en/ta/man/translate/figs-metaphor and rc://en/ta/man/translate/figs-metonymy)