What you need to do is have an extra scoreboard command (Warrow) and then go from there.

Put the command "execute @a[team=Shadow_mage] ~ ~ ~ scoreboard players set @e[type=Arrow,r=2] Sarrow 1" into a repeating command block with the following commands in order into 3 chain command blocks after it:

"/scoreboard players set @e[type=Arrow,score_Sarrow_min=1,score_Sarrow=1] Warrow 1 {inGround:1b}"

"execute @e[type=Arrow,score_Warrow_min=1,score_Warrow=1] ~ ~ ~ summon Wither ~ ~ ~"

"kill @e[type=Arrow,score_Warrow_min=1,score_Warrow=1]"

The first command finds out which arrows are shot by team Shadow_mage (or near them, unfortunately), and sets their score of Sarrow to 1, then the second command gives those arrows a score of 1 in Warrow when they hit the ground, the third command summons the Wither, and the last command kills the arrow to stop it from constantly summoning Withers

What you need to do is have an extra scoreboard command (Warrow) and then go from there.

Put the command execute @a[team=Shadow_mage] ~ ~ ~ scoreboard players set @e[type=Arrow,r=2] Sarrow 1 into a repeating command block with the following commands in order into 3 chain command blocks after it:

/scoreboard players set @e[type=Arrow,score_Sarrow_min=1,score_Sarrow=1] Warrow 1 {inGround:1b}

execute @e[type=Arrow,score_Warrow_min=1,score_Warrow=1] ~ ~ ~ summon Wither ~ ~ ~

kill @e[type=Arrow,score_Warrow_min=1,score_Warrow=1]

The first command finds out which arrows are shot by team Shadow_mage (or near them, unfortunately), and sets their score of Sarrow to 1, then the second command gives those arrows a score of 1 in Warrow when they hit the ground, the third command summons the Wither, and the last command kills the arrow to stop it from constantly summoning Withers