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Message: <blockquote data-quote="dave2008" data-source="post: 8261213" data-attributes="member: 83242">It is in the DMG guidelines:<h4>Effective Hit Points Based on Resistances and Immunities</h4> <table style='width: 100%'><tr><th>Expected Challenge Rating</th><th>HP Multiplier for Resistances</th><th>HP Multiplier for Immunities</th></tr><tr><td>1–4</td><td>× 2</td><td>× 2</td></tr><tr><td>5–10</td><td>× 1.5</td><td>× 2</td></tr><tr><td>11–16</td><td>× 1.25</td><td>× 1.5</td></tr><tr><td>17 or more</td><td>× 1</td><td>× 1.25</td></tr></table><h4>Step 15. Saving Throw Bonuses</h4>A monster with three or more saving throw bonuses has a significant defensive advantage, so its effective AC (not its actual AC) should be raised when determining its challenge rating. If it has three or four bonuses, increase its effective AC by 2. If it has five or more bonuses, increase its effective AC by 4.<h4>Monster Features</h4> <table style='width: 100%'><tr><td>Legendary Resistance</td><td>Ancient black dragon</td><td>Each per-day use of this trait increases the monster’s effective hit points based on the expected challenge rating: 1–4, 10 hp; 5–10, 20 hp; 11 or higher, 30 hp</td></tr></table>It is modified by the attack bonus, see rules text below, which I averaged to 17 (19 wand & 16 tail).  The expected attack bonus for a CR 21 attack is +11. SO 17-11 = 6/2 = +3 to offensive CR = 24Offensive Challenge Rating. Read down the Damage/Round column of the Monster Statistics by Challenge Rating table until you find your monster’s damage output per round. Then look across and note the challenge rating suggested for a monster that deals that much damage.Now look at the attack bonus suggested for a monster of that challenge rating. If your monster’s attack bonus is at least two points higher or lower than that number, adjust the challenge rating suggested by its damage output up or down by 1 for every 2 points of difference.It is what is in the DMG. You keep saying the official CR doesn't follow the DMG, but they generally do. You just have to look at the whole  DMG guidelines, not just the table.EDIT: I put quoted rules text in red</blockquote>

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