Once Sol'kanar the Tainted is controlled by your opponent, its ability triggers during that player's end step and that player makes all choices for it. That player draws a card for its first mode and gains life for its second mode, that player's opponents lose life for its second mode, and that player chooses one of their opponents for its last mode.

In a Two-Headed Giant game, Sol'kanar the Tainted's second mode causes the opposing team to lose 4 life. Its controller will still gain only 2 life.