Players can't gain life.
Damage can't be prevented.
All damage is dealt as though its source had wither. (A source with wither deals damage to creatures in the form of -1/-1 counters.)
Richard Kane Ferguson
The last ability affects all damage, whether itâ€™s dealt by creatures, other permanents, spells, or cards that arenâ€™t on the battlefield. Wither works everywhere.
The â€œdamage canâ€™t be preventedâ€ statement overrides all forms of preventing damage, including protection abilities. Damage prevention spells and abilities can still be cast and played; they just donâ€™t do anything.
Spells and abilities that would normally cause a player to gain life still resolve, but the life-gain part simply has no effect.
Spells and abilities that replace or redirect damage arenâ€™t affected by Everlasting Tormentâ€™s second ability. Theyâ€™ll work as normal.
If an effect says to set your life total to a certain number, and that number is higher than your current life total, that effect will normally cause you to gain life equal to the difference. With Everlasting Torment on the battlefield, that part of the effect wonâ€™t do anything. (If the number is lower than your current life total, the effect will work as normal.)
If a cost includes life gain (like Invigorateâ€™s alternative cost does), that cost canâ€™t be paid.
Effects that would replace gaining life with some other effect wonâ€™t be able to do anything because itâ€™s impossible for players to gain life.
Effects that replace an event with gaining life (like Words of Worshipâ€™s effect does) will end up replacing the event with nothing.