Shadow mastiffs are a nasty quasi-canine predators from the Shadowfell, valued as watchbeasts and hunting companions by the kinds of entities that would rather employ a monster for such purposes than pick up a nice puppy from the pound. Packs of them sometimes slip across the boundary between the Shadowfell and the material plane, roving and hunting for the joy of it.
With very high Strength and high Dexterity, shadow mastiffs are ambush attackers without the patience for a drawn-out fight. If they can’t take down their chosen target in two or three rounds of combat, there’s a good chance that they’ll give up and search for easier prey, and attacking from hiding is essential to their hunting pattern—they may not start a fight at all if they can’t gain surprise on the first round.
Five of their features—Shadow Blend, Sunlight Weakness, Keen Hearing and Smell, darkvision, and resistance to physical damage from normal weapons while in dim light or darkness—create such an overwhelming incentive for shadow mastiffs to stay out of sunlight and other areas of bright light that their entire hunting strategy revolves around exploiting the gloom of night. And since their Intelligence isn’t high enough for them to adapt to changing circumstances, lighting a torch or lantern or casting an illumination spell is an effective way for a target who survives their initial assault to get them to abandon their attack.
A shadow mastiff always begins combat invisible, having used Shadow Blend to hide in an area of dim light or total darkness (even if a player character has darkvision, dim light is enough!), and uses its Keen Hearing and Smell to detect prey. It’s worth noting that to the shadow mastiff, which has darkvision, darkness is effectively dim light, which normally would give it disadvantage on Wisdom (Perception) checks—but only those that rely on sight. If the shadow mastiff relies on its ears and nose instead, disadvantage flips to advantage, giving it an effective passive Perception of 18. Most player characters, and nearly all low-level PCs, will have actively use Stealth to slip past it, and it nearly always gets to surprise its opponents in round 1 of the combat encounter.
Like most predators, a shadow mastiff prefers an easy target: the young, old, weak, isolated and/or oblivious. But there’s a key caveat here: Bright light dispels its invisibility. So it’s never going to attack someone carrying a light source, either magical or mundane—although it can and will attack a target in the ring of dim light around that light source. As long as it stays in dim light or darkness, the shadow mastiff stays invisible, and as long as it stays invisible, it doesn’t have to meet or beat its target’s Perception check in a skill contest to gain advantage on its attack roll. (It does, however, have to meet or beat that Perception check with its own Stealth for its target to be surprised. PCs can hear things, too.)
After attacking a target that isn’t brightly illuminated, the shadow mastiff immediately uses its bonus action to vanish again with Shadow Blend, then exploits its invisibility to retreat a short distance away—generally just 10 feet, but occasionally up to 20—without provoking an opportunity attack. Instinctively, it moves in the opposite direction from the nearest light source; if there is no light source, it moves in a direction where it has a clear path back to its target, unobstructed by allies. Whenever possible, it ends its turn invisible and in an unpredictable location, so that opponents attempting to counterattack may aim at the wrong place and miss automatically.
The fact that the shadow mastiff is invisible doesn’t mean its opponents have no idea where it is—it does, after all, make sound. But its Stealth modifier is impressive: +6, reflecting expertise rather than mere proficiency. Stealth is a way of life for the shadow mastiff, and consequently, you can invoke passive Dexterity (Stealth) to say that even when it’s not taking the Hide action, it steps lightly, and passive Perception of at least 16 is necessary to pinpoint its location without Searching for it.
Its Bite attack includes a rider: a hit may knock the target prone. But DC 13 isn’t a high number to beat, a prone target will most likely just get up again when its turn rolls around, the shadow mastiff doesn’t need its target to be prone to gain advantage if it’s already invisible when it attacks, and it doesn’t have Multiattack. So we can safely disregard this aspect of its attack from a tactical standpoint. Other creatures fighting alongside one or more shadow mastiffs may be able to take advantage of a Bite that knocks the target prone, but the shadow mastiffs themselves gain no meaningful benefit from it.
The shadow mastiff’s Intelligence is very low, so its attack-vanish pattern never varies. Its above-average Wisdom indicates a solid self-preservation instinct and some ability to distinguish easier targets from harder ones; it prefers easier.
Predators often flee when they’re merely moderately wounded, disliking when prey fights back, but the shadow mastiff is an otherworldly beast that’s also neutral evil, giving it a measure of innate truculence that keeps it in the fight until it’s seriously wounded (reduced to 13 hp or fewer). It’s more likely to be driven off by bright light than by injury.
It’s fair to assume that a shadow mastiff’s targets will be surprised in round 1. It continues to attack in round 2 even if its target is brightly illuminated—unless the entire area is flooded with bright light, leaving it no area of shadow to retreat to, in which case it bolts. In round 3, it won’t attack any target in bright light, and it retreats if this leaves it without any attractive target. If it hasn’t taken out a victim in three rounds, it retreats in round 4. It always Dashes when retreating, turning invisible as soon as it’s out of the light.
Ethereal Awareness is a bit of a nothingburger. It can see creatures in the ethereal plane, but that doesn’t mean it can attack creatures in the ethereal plane. Maybe, however, there’s something about opponents that approach via the ethereal plane (or who cast an evasive spell such as blink) that makes them more attractive to it. I wouldn’t think about it too hard.
The variant shadow mastiff alpha differs in only one significant respect from other shadow mastiffs: Terrifying Howl, which it uses in round 1 for maximum effect. Like dragons’ Frightful Presence, there’s no reason to use it more than once: if it fails, it will never work, and if it succeeds, it won’t work again once it wears off. It doesn’t confer direct advantage (the frightened condition only makes it harder for affected creatures to fight back), so it doesn’t change the tactics of the alpha or any other shadow mastiff in any way.