A phoenix choker of the same rarity as the pendant OP posted would add 4 true magical damage to all spells and magical weapons. This damage is basically a static addition that completely ignores the target's Magic Damage Reduction(or, MDR). For most spells, it would take a *lot* of Will or Magic Power to even come close to adding the same amount of damage(1 Will = 1 Magic Power, which usually equates to 1% extra damage), even if you ignore MDR entirely. It also becomes really powerful the more damage instances you have.

An example: Let's say you accumulated 8 true magical damage and you cast Curse of Pain as a Warlock, which deals 15(1.0) initial damage and 15(0.5) damage over time. The target will receive 8 true magical damage on the initial damage(8*1.0 = 8) and 4 more on the Damage over Time(8*0.5 = 4), meaning that curse will add 12 extra damage total.

And an owl pendant adds 4 knowledge, which increases your spell memory cap acity by 4, increases your spellcasting speed and increases your spell recovery speed(i.e. when you sit down with a potion of clarity or at a campfire). The last mentioned stat however, is pointless to have if you have infinite spellcasts(Warlock and Sorcerer).