The devas are a group of angelic beings added in SLASH'EM. They are all characterized by strong physical attacks, high speed, and clerical spellcasting, similar to aligned priests. They are most often encountered as minions, either given to lawful characters through prayer or sacrifice, or created when attempting to convert a lawful altar. As minions, they are quite powerful, and can be a useful source of shields of reflection or even artifact weapons, which they have a chance of starting with.
The movanic deva is the weakest of the devas. It is still a formidable opponent, particularly if one lacks fire resistance, as its attacks will then burn you. It is able to cast clerical spells, including possibly curse items and summon insects.
Monadic devas are slightly stronger than movanic devas, and are distinctive in that their physical attacks do only normal damage. Still, they are quite formidable opponents, similar in level to an Angel but much faster. They are also able to cast all clerical spells, including fire pillar (same effect as a scroll of fire) and geyser.
The toughest of the devas, astral devas are almost equivalent to an Archon in difficulty. They do similar amounts of physical damage, and while the astral deva cannot stun and blind with its gaze, it can stun with its physical attacks. The astral deva is also slightly faster than the Archon, with a speed of 18 over the Archon's 16. They are almost the same base level (18 for the astral deva versus 19 for the Archon), and are both spellcasters. Luckily, the astral deva is unable to summon nasties, as it can only cast clerical spells, which summon nasties fortunately is not. It can, however, summon insects, which is somewhat dangerous given some of SLASH'EM's new insects, such as the assassin bug or killer beetle.
The devas, with the Planetars and Solars, constitute one of the chief reasons that lawful alignment is very nice in SLASH'EM. Not only are these dangerous monsters always peaceful to you, but you have a very good chance of receiving a number of them as pets, both from altars and occasionally from praying while in trouble. Additionally, receiving them as minions from sacrificing does not increase your prayer timeout, and so does not even require that much time or nutrition.
If you are not lawful, these are all formidable opponents. It should be noted that, "in the wild," these monsters are all quite rarely seen, particularly since they cannot appear in Gehennom and are relatively high-level. However, one should think twice about converting lawful altars in SLASH'EM. It is better to convert altars at lower experience levels, since then easier couatls and Aleaxes will be created instead of devas. It is best, in fact, to convert altars below experience level 4, since then no minions will be created.
In combat, it is good to keep in mind that devas are not guaranteed a shield of reflection, unlike Archons, although they do have a reasonable chance of having one. They all resist death rays, but no other types of damage. Thus, attack wands work well against them, although their high MR of 90 means that wands of sleep, striking and magic missile are likely to be ineffective. If they do have a shield of reflection, wands of draining and wands of fireball are good choices against them, since they resist neither and neither can be reflected. Stoning is also a good choice if it is available; since they do not summon nasties, however, a cockatrice corpse is less likely to be around. Escape items, in particular wands or scrolls of teleportation, are recommended as a back-up plan, in light of their high speed and the fact that they will follow you from level to level if adjacent to you.
Devas can make very powerful pets. They are very fast, can heal themselves, don't eat, and unlike the most powerful pets, are medium-sized and can thus wear armor and cloaks. This can be particularly useful to a pacifist character, as it is much easier to give devas magic resistance, either with a cloak of magic resistance or gray dragon scale mail. Contrast this to large monsters, which can only gain extrinsic magic resistance by wielding Magicbane. The immunity to polytraps and level-teleportation traps devas thus can gain can make them much more reliable pets. Also, an astral deva wearing a fully-enchanted set of armor will have an AC of around -59, about the best achievable for any monster. Still, an astral deva can only reach level 27 without potions of gain level; while it will be powerful and difficult to kill, it still will not engage the tougher monsters in SLASH'EM without conflict.
One should note that while useful, devas gained as minions through sacrifice or prayer cannot pick up or use any items other than the ones they start with. In particular, this means that disarming one (perhaps because it is carrying Sunsword) will disarm it permanently, somewhat decreasing its damage potential, and making it unable to attack petrifying monsters at all. However, for lawfuls, it is possible to gain large numbers of these minions, who in combination with a magic whistle can be very useful.