Strength

Strength is a buff that increases damage dealt by Attacks. Each point of Strength gives +1 damage per hit.

Bonus damage from Strength is added to the calculation first before other modifiers such as Vulnerable, Weak, Wrath, or Divinity.

Both the player characters and many enemy types are capable of gaining Strength.

It is possible to have negative stacks of Strength. For every negative stack, damage dealt is reduced by one per hit.

Strength is capped at 999 and -999.

Sources[]

Vajra - 1 Strength.
Girya - Adds the Lift option to Rest Site. Use to gain a permanent 1 Strength, up to 3 times per run.
Shuriken - 1 Strength for every 3 Attack cards played in a single turn.
Du-Vu Doll - 1 Strength for each Curse in your deck.
Sling of Courage - 2 Strength at the start of each Elite combat.
Mutagenic Strength - 3 Strength at the start each combat that is lost at the end of your turn.
Brimstone - 2 Strength to the player and 1 Strength to all enemies at the start of your turn.
Red Skull - 3 Strength while your HP is below 50%.

Lagavulin - Reduces Dexterity and Strength by 1 (2 ¹⁷) when using its Siphon Soul.
Spire Shield - Reduces Strength by 1 when using its Bash (50% chance if player has at least 1 orb slot).

Lethality modifier in Custom Mode: You and ALL enemies start each combat with 3 Strength.

Cultist - From its Ritual power: Gains 3 (4 ²) (5 ¹⁷) Strength at the end of every turn.
Jaw Worm - Gains Block and 3 (4 ²) (5 ¹⁷) Strength when using its Bellow. In act 3, all Jaw Worms gain the benefits of Bellow before the player's turn 1 begins.
Red Louse - Gains 3 (4 ¹⁷) Strength when using its Grow.
Fungi Beast - Gains 3 (4 ²) (5 ¹⁷) Strength when using its Grow.
Mad Gremlin - From its Angry power: Gains 1 (2 ¹⁷) Strength when taking attack damage.
Byrd - Gains 1 Strength when using its Caw.
Chosen - Applies Weak and gains 3 Strength when using its Drain.
Centurion & Mystic - ALL enemies gain 2 (3 ²) (4 ¹⁷) Strength when Mystic uses its Buff.
Darkling - Gains 2 Strength when using its Harden on Ascension 17 and above. Loses all Strength when its HP reaches 0, relevant for when it revives with its Reincarnate.
Orb Walker - From its Strength Up power: Gains 3 (5 ¹⁷) Strength at the end of every turn. (Gets no additional Strength at Ascension 2.)
The Maw - Gains 3 (5 ¹⁷) Strength when using its Drool. (Gets no additional Strength at Ascension 2.)
Transient - From its Shifting power: Upon losing HP, loses that much Strength until the end of the turn.

From the Elite Buffs given by the Emerald Key: 25% chance for a Super Elite to have additional Strength, equal to the current Act number + 1.
Gremlin Nob - From its Enrage power: Gains 2 (3 ¹⁸) Strength whenever you play a Skill.
Gremlin Leader - Gains Block and 3 (4 ³) (5 ¹⁸) Strength when using its Encourage.
Taskmaster - Deals damage, shuffles Wounds, and gains 1 Strength when using its Scouring Whip on Ascension 18 and above (has no other moves, so applies every turn).
Spire Spear - ALL enemies gain 2 Strength when using its Piercer.

Hexaghost - Gains Block and 2 (3 ¹⁹) Strength when using its Inflame.
Bronze Automaton - Gains Block and 3 (4 ⁹) Strength when using its Boost.
The Champ - Gains 2 (3 ⁴) (4 ¹⁹) Strength when using its Gloat. Clears all debuffs and gains 6 (9 ⁴) (12 ¹⁹) Strength when using its Anger (the turn after Champ drops below 50% HP).
The Collector and Torch Head - ALL enemies gain Block and 3 (4 ⁴) (5 ¹⁹) Strength when Collector uses its Buff.
Awakened One - Starts combat with 1 Strength at Ascension 4 and 2 Strength at Ascension 19. From its Curiosity power: Gains 1 (2 ¹⁹) Strength whenever you play a Power card. Keeps all current Strength after reviving with its Rebirth.
Time Eater - From its Time Warp power: Ends your turn and gains 2 Strength every 12 cards you play this combat.
Donu and Deca - ALL enemies gain 3 Strength when Donu uses its Circle of Power.
Corrupt Heart - Removes all negative Strength from itself, then gains 2 Strength plus an additional buff when using its Buff. This additional buff is 10 Strength at turn 13, and 50 Strength at turn 16 and every 3rd turn after.

Philosopher's Stone: ALL enemies start each combat with 1 Strength.
Lethality modifier in Custom Mode: You and ALL enemies start each combat with 3 Strength.

Weekly Patch 51: Recursion
- Fixed issue where Jaxxed achievement was obtained when enemies gained 50 Strength.
- Negative Strength power typo fix.
Weekly Patch 50: The Final Act
- Red Skull relic's Strength gain is now correctly applied to the cards in your hand at start of combats.
Weekly Patch 48: Creative AI
- Certain enemies no longer gain extra strength when removing Piercing Wail or Dark Shackles from themselves.
Weekly Patch 44: Chrysalis
- Hotfix: BUG FIX: Red Skull relic now correctly loses bonus strength when healing above 50% HP.
Weekly Patch 42: Reboot
- Fixed issue where upgraded card from Warped Tongs wasn't showing the correct value if you had Strength/Dexterity.
Weekly Patch 22: Testing the Third Character
- Localization: Strength and Dexterity power can now properly utilize localized periods.
Weekly Patch 6: We're Back!
- Corpse Explosion is now affected by Powers (Vulnerable, Strength, Pen Nib, etc)
Weekly Patch 2: Slimes!
- Strength and Dexterity now cap at 999 and -999.