Scroll of the Beast

Gives friendly nearby units a 25% bonus to damage for 45 seconds.

Source
Arthas and Muradin Bronzebeard find one in The Shores of Northrend, chapter 7 of the Warcraft III human campaign, The Scourge of Lordaeron.

Cairne Bloodhoof can find one in The Oracle, chapter 7 of the orc campaign, The Invasion of Kalimdor.

Illidan can find one in A Destiny of Flame and Sorrow, chapter 6 of the night elf campaign, Eternity's End.

Kael can find one in Misconceptions, chapter 1 of the Frozen Throne alliance campaign, Curse of the Blood Elves.

Illidan, Kael, Lady Vashj and Akama can find one in Lord of Outland, chapter 6 of that campaign.