[3.5] The Rogue Handbook: A Fistful of d6

[Curmudgeon]

The Darkness Invocation grants you Darkness the spell, as a spell-like ability.

That doesn't make any sense. These are different types of abilities, with different rules.

Invocations: A warlock does not prepare or cast spells as other wielders of arcane magic do. Instead, he possesses a repertoire of attacks, defenses, and abilities known as invocations that require him to focus the wild energy that suffuses his soul.
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A warlock’s invocations are spell-like abilities; using an invocation is therefore a standard action that provokes attacks of opportunity.
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Since spell-like abilities are not actually spells, a warlock cannot benefit from the Spell Focus feat.

The Darkness invocation simulates the Darkness spell, but it's always a SLA rather than a spell.