Threat of Thassarian

Threat of Thassarian is a talent in the Frost tree, you will need to spend 38 points in Frost to max it.

This is a key talent that makes dual wielding viable for death knights. Normally only the main hand weapon is taken in to account for abilities that deal damage based on weapon damage, such as Blood Strike and Obliterate, severely reducing their damage when dual wielding instead of using a 2-hander. This talent 'fixes' that problem for deep frost death knights. Any death knight wishing to dual wield must max this talent; doing otherwise makes attacks hit half as hard as they should be.

Blood Strike, for example, hits for 40% weapon damage plus a constant based on attack power, and bonus damage per disease. A 2-handed weapon, like has for 651-977 unmodified weapon damage. 40% of that is 260-390 damage. A 1-handed weapon of the same item level,, has 325-625 unmodified weapon damage. 40% of that is 130-242 damage. It also gains less damage from each point of attack power (see normalization). From here, it should be clear that in order to dual wield as a death knight, your strikes must be hitting with both weapons.

Trivia
This talent refers to Thassarian, the first death knight to rejoin the Alliance.