The full list of tryouts for the Raiders includes:
Darkwa, 26, originally signed on with the Dolphins as an undrafted free agent out of Tulane back in 2014. He spent just a few months in Miami before he was signed off of their practice squad by the Giants.
New York brought Darkwa back on an exclusive rights contract in 2016 and re-signed him to a one-year contract last year.
In 2017, Darkwa appeared in 15 games for the Giants and rushed for 751 yards on 171 carries (4.4 YPC to go along with 19 receptions for 116 yards receiving and five total touchdowns.
Allen, 22, signed on with the Panthers as an undrafted free agent back in April. He began his college career at Texas A&M before transferring after two seasons and joining Houston for the 2017 season.
Allen was waived coming out of the preseason before the Panthers added him to their practice squad last week, but elected to waive him last week in favor of DT Kendrick Norton.
During his college career, Allen threw for 4,283 yards while completing 61.7 percent of his passes to go along with 37 touchdowns and 18 interceptions over the course of three seasons and 24 total games played.
Callahan, 25, wound up signing a three-year, $1.62 million contract with the Packers as an undrafted free agent out of Wesley College back in May of last year. He was able to make the 53-man roster as the team’s No. 3 quarterback coming out of the preseason but was later cut loose.
Callahan had brief stints with the Saints and Browns before returning to the Packers during the 2016 season. Green Bay cut Callahan loose as they trimmed their roster down to 53 players and he was later signed to their practice squad.
The Packers re-signed Callahan as an exclusive rights free agent during the offseason, but he was let go and later signed by the Eagles. Unfortunately, Callahan was among their roster cuts coming out of the preseason.
In 2017, Callahan appeared in one game for the Packers and completed five of seven pass attempts for 11 yards and no touchdowns of interceptions.