The Altars of Sorrow is a relatively recent addition to Destiny 2, and the weapons you can get from it are, for the most part, very good. In this guide I’ll give an overall strategy and then subclass picks for each class. (Do note that I play Warlock much more than Titan or Hunter, so everything I say may not be the best choice.)
The Altars of Sorrow is a wave-based mode much like Blind Well or Escalation Protocol. The main idea is to kill enough Nightmares before the time runs out to advance to the next wave, with there being five waves plus a boss wave. Do not let Nightmares reach the altars! If they do it subtracts 10 seconds off the clock, which can be game over on some of the higher waves. Don’t ignore the adds either, because killing them adds 1 second to the clock for each one.
Starting on wave 3, you will need to kill Wizards to spawn more Nightmares. These Wizards have invincible shields that can only broken by Hive swords, which are found on Knights wandering around the altars. Try to clear as many of the adds around the altar as you can before you pick up the sword, as you can easily be overwhelmed.
After wave 5 you will fight a boss, which is level 960 and can kill you quickly if you’re at a lower light level. After damaging the boss enough it will enter an invulnerability phase, and Nightmares will start heading toward the altar. Make it your top priority to kill the Nightmares here, as you want as much time on the clock as you can. I’ve failed this quite a few times because people kept damaging the boss and ignored the Nightmares, making us lose all our time.
Warlock: There are two that I like to use for this, Devour and Chaos Reach, while Well of Radiance is also very good.. Devour will work well no matter what exotics you have equipped, but if you want to become an almost invincible avatar of destruction equip Nezarec’s Sin and something like the Recluse. For Chaos Reach it’s pretty much mandatory that you have Geomag Stabilizers equipped, and along with Bad Juju you will almost always have your super. With Well slap on Phoenix Protocol and go to town on everything.
Titan: Thundercrash with Insurmountable Skullfort is very good for add clear, but isn’t that great for damaging Nightmares. If you like watching your enemies disappear in massive purple explosions, use Code of the Commander with Oppressive Darkness and Voidwall grenades. Unfortunately Oppressive Darkness won’t be around for much longer so enjoy it while you can. Burning Maul with Synthoceps is good for both boss damage and add clear.
Hunter: Both Tethers work well here, for both boss damage and add clear, and Blade Barrage does the same. Other than that, I can’t think of much else besides equipping Liar’s Handshake and punching everything, which if you haven’t tried it is a lot of fun.