Tibia Mariner's AI is extremely simple. It attacks slowly and, for the most part, thoughtlessly with only one set of moves that could charitably be described as a combo.
Despite this, it posed some unique challenges in determining how it worked. This is the first boss I've looked into that uses magic heavily, and one of the major downsides of the DS Animation Viewer tool I use is that it can't even visualize magic, let alone provide clear insights on what it does.
Tibia Mariner had two magical attacks in particular that I found puzzling: one where it sets its horn on the prow of the ship and blows, and another where it blows the horn at a forty-five degree angle up into the air. I strongly suspected that these were connected in some way to its ability to summon skeletons to aid it in battle, but precisely how was unclear.
To solve this mystery, I enlisted the aid of my brilliant and talented wife @JhoiraArtificer, who had already been helping me out with some wildlife photography. I asked her to travel around to the four Tibia Mariners scattered across the map and try to figure out how they worked—and particularly if they worked differently from one another.
The first thing she realized was that only the first two Mariners you encounter, in Limgrave and Liurnia, actually summon Skeletal Militiamen—and it does so using its low-horn attack. Although there are Skeletal Militiamen around the Mariner in Altus Platea, they're just features of the area itself and aren't resummoned by the Mariner if it's killed. Instead, it uses the high-horn attack to summon a single giant skeleton to emerge from the ground and attack you.
The final Mariner in Mountaintops of the Giants is even stranger. It's surrounded by giant skeletons much like the one in Altus, but these skeletons are more autonomous—they don't wait for the high-horn signal to attack at all. But it spawns in a little nook where the skeletons can't reach! So all in all it's probably the easiest to defeat.
Once we figured this out and jotted down the meaning of each of the horn positions, @JhoiraArtificer and I ran another experiment. Normally it's very difficult to kill the giant skeleton summoned in Altus, because it emerges from the ground briefly in an unpredictable location and is quite dangerous while it's visible, so it's hard to land more than a hit or two. But I'd hacked the game to give Liz's character (among other useful tools) an ivory sickle that instantly kills anything it hits. So what happens if you kill this skeleton?
It turns out that after it dies, the Tibia Mariner does a low-horn, and then is able to call on the skeleton again with further high-horns. Now I understood how they worked: each Mariner has, hard-coded somewhere I never found, a list of skeletons it specifically summons every time it does a low-horn. The Altus Mariner's list is one entry: the giant skeleton. That skeleton then spends most of its time under the map, until it hears the high-horn and lunges out to attack. But fundamentally, it's the same system as the first two Mariners: if your allies are dead, summon them again, otherwise just focus on attacking!
