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To use this spreadsheet: Make a copy by File > "Make a copy", and fill in your own values in the green cells
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Written guide with more details on mechanics, strategies, and gearing: https://royalgiraffe.github.io/leotheras
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Link to latest version of this sheet: royalgiraffe.github.io/leotheras-calc
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More of my WoW guides and calculators: royalgiraffe.github.io
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In a single Demon Form phase, Leotheras will typically cast Chaos Blast about 17-20 times. So to tank the Demon Form, you need a tank with high fire resistance to resist most of the casts, since depending on your HP you'll be one-shot if 8-10 Chaos Blasts hit you.
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This calculator compares the chance of death with different FR and HP. This calculates the probability of dying to Chaos Blast in a single Demon Form phase - of getting hit enough times that the stacking damage is enough to one-shot you.

There are several factors that this doesn't account for, such as:
- You can drop stacks by having a Paladin cast Divine Intervention on you, and Paladin tanks can Divine Shield themselves. This is great for in case you get unlucky and are about to reach your stack limit.
- You can survive an extra hit with a shield/absorb timed right after you get to enough stacks that the next hit will kill you. Options include Power World: Shield and Major Fire Protection Potion. Warlocks can FelDom a Voidwalker for the Sacrifice shield. You could probably even survive 2 extra hits with careful usage and timing.
- Other cooldowns can boost your effective HP pool temporarily, such as Nightmare Seed (consumable on shared cooldown with healthstone and Frozen Rune) and Pain Suppression (Discipline priest spell)

On the other hand:
- There's also a chance of dying if you get hit twice in a row and don't get healed enough in between
- There's usually not enough time to drop the debuff between the last Demon Form phase and the split phase, so you'll just keep building up stacks, and the number of casts can go far beyond 20. (The debuff lasts 45 seconds, so your stacks don't drop until near the end of Human Phase.) If you're close to dropping your debuff, consider having someone else take the first couple hits, since it doesn't start doing big damage until you take several stacks.
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Compare different sets here
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Total (buffed) FR365365363350335
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Total (buffed) HP12,00013,50013,50013,50015,100HP thresholds
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How many hits you can survive before being one-shot78889Num Chaos Blast hitsMinimum HP to survive
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Chance to hit (includes FR and 1% miss chance)24.8%24.8%25.2%27.8%30.9%58,376
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Chance of being one-shot with 20 Chaos Blast casts9.7%3.9%4.2%7.5%5.7%610,051
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711,726
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Num Chaos Blast castsProbability of getting one-shot813,401
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162.6%0.7%0.8%1.6%0.9%915,076
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173.8%1.2%1.3%2.5%1.6%1016,751
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185.4%1.8%2.0%3.8%2.6%1118,426
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197.4%2.7%3.0%5.5%3.9%1220,101
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209.7%3.9%4.2%7.5%5.7%
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2112.4%5.3%5.8%10.0%8.0%
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2215.5%7.1%7.7%13.0%10.7%
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This chart shows the chance of getting one-shot killed with 20 Chaos Blast casts, for different amounts of HP and FR. So for example, you can see that you have a 10% chance of death with about 350 FR and 13401 hp, or with about 325 FR and 15076 hp.
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