Herald of Xotli: Combo Rotations

PVP: When deciding on a rotation for PVP all you really need to do is keep an eye on the cooldowns of the key abilities for the class your facing. This usually means watch the timer for Burn to Death, Hellfire Breath and Fire Lance should be watched. You also need to take into account the situation you're in as you won't always want to spend the time on Burn to Death when direct damage will get the job done and you won't usually want to waste a solid Fire Lance if you know the person will be able to quickly recover from it (ie a powerful, high cd heal), but you will want to use Hellfire Breath on every possible cooldown. No real secret to how you should use combos in PVP, just balance damage with survivability as much as possible.

PVE: When picking a combo rotation you will need to take a number of aspects into consideration, such as the mob type (boss, minion, elite, regular, etc.) and how much mana and/or stamina conservation you need. Generally speaking, though, your mana should be fine unless you spend alot of time debuffing and such. You will also want to take threat/hate into account as well, which is where both damage control and Molten Steel Slash come in to play. On top of all of this, don't forget you can weave lower ranking combos into your rotation to help work with the cooldowns. Now onto what I generally use for rotations..

Elites (single or two), the ones you will find in your average group instance, are generally quick to die and thus require a small rotation, something along the lines of Max Molten Steel Slash > Hellfire Breath > Hell Strikes > 2nd Highest Molten Steel Slash > Hellfire Breath > 2nd highest or other strong ability (note that I'd only really recommend the 2nd highest Hell Strikes at 80). Be sure to position yourself so you hit both mobs if there are two, of course, and this especially goes for the Molten Steel Slashes for when you move to that mob. You can feel free to use Desecrating Essence/Avatar and Exultation of Xotli at will, but general debuffing is not necessary.

Elites (Three +): Groups are something Heralds are naturally good at damaging, and this can often result in you pulling aggro in a situation like this if you have an insufficient number of tanks and therefore you should plan ahead. You can go through a large group in one of two main ways, you can either follow the single/double elite rotation or you can add in Pillar of Flame, your most powerful polymorph (desecrating or avatar) and any other defensive skills you may have such as Phoenix Cloak. But before you choose your path be sure that a. your group can keep you alive through healing/tanking or b. your group can help damage them down before you die (or c. you can solo them before you die). If you choose to burn them down 1 by 1, debuffing isn't frowned upon, and Exultation of Xotli can help on the last mob or two, but using Desecrating Essence/Avatar is most likely a bad idea.

Bosses: You will want to certainly use Burn to Death as it results in a large amount of overall damage and the bosses will always live long enough for the Burn to Death effect to actually kick in. Giving the tank a lead on hate is a common practice in this case so using Molten Steel Slash more than once or twice is most definitely overkill, instead add in Burn to Death and Pillar of Flame/Firelance or something else to take over those spots in the rotation. Pretty much use the single Elite rotation for the first rotation or two and move to some form of Hell Strikes > Hellfire Breath > Burn to Death > Hell Strikes > Hellfire Breath > Pillar of Flame. Note any resistances or invulnerabilities the Boss may have, for example: cut out some or all of your fire combos for a fire resistant boss, or add more for a frost based boss. The same goes for focusing more on magic for bosses highly invulnerable to melee and vice-versa.