Dragon Slayer and Demon Slayer codex unlocks in RS3 apply to the bosses that count as those creatures for Slayer purposes, but not to every boss with a similar theme. In practice, that means they work on boss variants tied to dragons or demons, while regular bosses that only happen to use dragon or demon visuals usually do not count.
Dragon Slayer codex
The Dragon Slayer-related unlock is used for bosses that are treated as dragons, so it benefits fights against dragon-type bosses such as Elvarg-style content and other bosses that are explicitly classified as dragons in the game’s Slayer system. It does not automatically apply to every boss that breathes fire or looks draconic. For Slayer tasks, the important distinction is whether the boss is actually assigned as a dragon-type target. If the boss counts as a dragon, the codex bonus can matter; if it does not, the unlock has no effect.
Demon Slayer codex
The Demon Slayer-related unlock works the same way, but for demon-classified bosses. It is relevant on bosses that are specifically treated as demons for Slayer, including demon bosses and boss variants that are flagged as demons rather than merely being demon-themed. That means it can help on demon-type boss slayer tasks and on bosses that the game explicitly recognizes as demons, but not on every enemy with a demonic appearance or lore.
The practical rule
A simple way to judge it is this: if the boss is a genuine dragon or demon in the Slayer classification, the codex works. If it is just dragon-like or demon-like, assume it probably does not. That distinction matters because RuneScape often separates creature theme from creature type. A boss can look like a dragon or demon and still not count for the codex effect.
Best way to check
If you are trying to use these unlocks efficiently, check the boss’s actual Slayer classification before you plan a task around it. The game’s classification is what matters, not the name alone.