Don’t let the neotenic proportions in the illustration in Volo’s Guide to Monsters fool you: devourers are big and mean enough to pick you up and stuff you inside their own ribcages. Which they do. It’s a thing.
Fiends, though not technically demons, devourers seize humanoids and consume them body and soul, transforming them into undead creatures of power proportional to what they possessed in life.
Devourers have extraordinary Strength and Constitution; their Charisma is also very high, but they’re melee-oriented brutes first and foremost. With above-average Intelligence, they’re going to be fairly good at guessing who’s going to be susceptible to which of their abilities, though these guesses are by no means infallible. With 120 feet of darkvision, they’re not creatures you’re ever going to encounter in broad daylight—strictly at night, indoors and/or underground. (According to the flavor text, they aren’t even found on the material plane all that often.)
Since their Multiattack includes a use of Imprison Soul or Soul Rend in addition to their melee attacks, and since Soul Rend’s area of effect is centered on them, there’s no reason for them not to charge straight into combat. In fact, Soul Rend synergizes with melee combat, punishing opponents for engaging with it toe-to-toe. Despite its slow Recharge (only on a roll of 6), the devourer has every incentive to use it whenever it can—if it can affect at least four opponents within the 20-foot radius. Fewer than that, not worth it, and its armor class and damage resistances aren’t quite good enough for it to be willing to risk more than one opportunity attack to reposition itself just to affect a fourth enemy with Soul Rend. Five or six? Play it by ear.
Imprison Soul is easy to misconstrue and requires a couple of close reads to understand what it does. “The devourer chooses a living humanoid with 0 hit points”—in other words, this ability functions only against opponents who have already been knocked out and are about to start making saving throws vs. death. It has no effect on an opponent who’s wounded but still fighting. Therefore, the devourer’s first priority is to get an opponent down to 0 hp.
For this reason, once the devourer picks an opponent, it’s going to singlemindedly focus all its attacks on that one opponent. When making its initial selection, it’s going to go for the softest humanoid target it can easily get at (it ignores nonhumanoids completely). It doesn’t have any special movement ability, so it may have a hard time reaching a squishy spellcaster in the back line—although if one has thoughtlessly mispositioned him- or herself within easy reach, you as the dungeon master are contractually obligated to punish that PC’s mistake.
A devourer can’t “read” PCs’ stats the way extraordinarily intelligent monsters can, but you can assume it can tell the difference between a PC with a Constitution of 16 or more and one with Con 10 or 12, and between one with Con 10 or 12 and one with Con 7 or 8. You can also assume it knows that dwarves are somewhat tougher than other races on average, and of course it knows that heavy armor is hard to hurt someone through.
As an example, let’s look at the primary party I’ve been DMing for: a human barbarian with Con 17 and no armor; a halfling ranger/rogue with Con 9 and light armor; a human paladin with Con 12 and heavy armor; a dwarf druid with Con 15 and medium armor; a human bard with Con 16 and light armor; and a dwarf ranger with Con 13 and light armor. Out of this lineup, the devourer would immediately rule out the barbie (clearly much too tough), the pally (too well-armored) and the druid (just a little too difficult in too many ways). Of the remaining three, the halfling looks like the easiest target, followed by the dwarf ranger and the bard. But the bard is a supporter, while the halfling is a sniper and the dwarf ranger is a skirmisher. The devourer would probably find the bard easiest to get at; if so, it would target him, despite his high Constitution, rather than the halfling or the dwarf ranger.
Having chosen a target, the devourer beelines for him or her and immediately makes two raking claw attacks. If there are at least four enemies within 20 feet—or if there are fewer than four enemies altogether, and they’re all within 20 feet—it also pops Soul Rend immediately. If not, it holds off on Soul Rend until that criterion is met.
Beyond this, it continues to viciously attack the same target with its claws and, if applicable, Soul Rend, until its target is reduced to 0 hp. At this point, it now knows roughly how much damage it had to deal to take its target down. It doesn’t want to kill the target before it can use Imprison Soul against it, but if it can safely make another claw attack in order to force its target to fail a couple of death saves before getting sucked into its torso, it will, because that makes its job easier. Thus, if the target had 100 hp or more at the start of the combat encounter, the devourer makes one more gratuitous claw attack against him or her (advantage, auto-crit on hit). Then it uses Imprison Soul against the target and moves on to deliver any remaining claw attack(s) against its next priority target.
Imprison Soul is an automatic ability: there’s no roll to hit and no saving throw against it. The death saves take place on the target’s turn, and—this is crucial—there’s no way to get the target out of the devourer, aside from, implicitly, destroying the devourer. There’s also no way to reach the target to deliver a touch-required healing spell.
What if an ally uses, say, healing word against the target? As far as I can infer, he or she is revived—but still trapped inside the devourer’s body. Can the devourer reach inside itself with its claws to attack a revived prisoner? Based on the illustration (and with nothing else to go on), yeah, I’d say so, but since the stat block says nothing about the imprisoned creature’s being restrained, the devourer doesn’t have advantage on this attack, and I’d say the prisoner probably even has half cover from the devourer’s own ribs. However, Soul Rend definitely affects any imprisoned creature, conscious or not, and you should count the imprisoned soul toward the four-opponents criterion for using this ability.
When the victim fails his or her last death save, the devourer immediately regains 25 hp and the use of Soul Rend, although it can’t use the latter until its turn comes around. At that point, it vomits out its victim, who has now become a wight (if you’re throwing a devourer at PCs of level 5 or lower, you’re guilty of DM malpractice) that’s really cheesed off that its erstwhile allies let this happen to it and that takes out its resentment against an opponent whom the devourer passed up as too hard a target. The devourer also gains an additional action, which can be another Multiattack! This is a cascade failure situation for the devourer’s opponents. Seriously, don’t throw a devourer at your PCs unless you’re confident that they have their act together enough to head off a TPK. In fact, maybe wait till they’re at least level 11 to even consider it. Maybe even give them some advance warning in the form of lore about what devourers do, so they know how seriously to take the threat.
Devourers are both fiends and fanatics. Even though they don’t respawn in the Abyss upon being destroyed on another plane, as demons do, they’re not going to run away, no matter how much damage they’ve taken.