Truesight and Darkvision, why does a monster have both?

Question

While creating a homebrew monster based around eyes and vision, I looked up monsters that had both darkvison and truesight, surprisingly only two have both, the Avatar of Death and Canoloth, I'll use the Canoloth as the example here.

When reading the descriptions of both vision types, darkvision allows a creature to see in dim light as if it were bright light and darkness as if it were dim light but it can't discern color and only sees shades of grey, with truesight not only can you see in normal darkness but also magical darkness, as well as many other benefits, so what I want to know is what mechanical reason would a creature need to have both forms of vision (especially when it only has darkvision out to 60 feet but truesight out to 120 feet) when truesight already has the only benefit of darkvision along with all its other benefits?

Is their an actual mechanical benefit to give a creature like the Canoloth both forms of vision?

Answer 1

For all those canoloths with ranger levels (and goggles)

If a canoloth were to take three levels of ranger¹, the Gloom Stalker subclass provides the Umbral Sight feature:

At 3rd level, you gain darkvision out to a range of 60 feet. If you already have darkvision from your race, its range increases by 30 feet.

The canoloth now has darkvision out to a range of 90 feet. That's still less than the range of its truesight, so it will need to invest in some goggles of night:

While wearing these dark lenses, you have darkvision out to a range of 60 feet. If you already have darkvision, wearing the goggles increases its range by 60 feet.

The canoloth's darkvision now extends to 150 feet, beyond the reach of its truesight. If it did not originally have darkvision, Gloom Stalker and goggles of night would only grant it darkvision out to 120 feet.

But yes, it's pretty niche

Darkvision does not provide any mechanical benefits over truesight. While there are means of evading the true seeing spell (like nondetection), those don't work against innate truesight.

Every yugoloth in Mordenkainen's Tome of Foes (including the canoloth) has 60 ft. darkvision. It would be reasonable to assume that darkvision is granted to all yugoloths, just like it is granted to all elves. Subraces can then modify those senses - canoloths gain truesight, while drow gain extended darkvision.

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¹ There are rules in the DMG (p. 283) for giving monsters class levels.

Answer 2

There may be a way to remove one or other of the extra senses (though I can't find one offhand that would affect only one but not the other). If a monster only had truesight and not darkvision, removing the ability to see through illusions and onto the Ethereal Plane would also remove the ability to see in the dark. If a creature has both senses it could lose the truesight but still be able to see in the dark.

In the particular case of the Canoloth, the ranges are different. It has darkvision 60 ft and truesight to 120 ft. If you somehow manage to defeat the truesight but not the darkvision, not only have you removed some of its extra senses, but you have changed the range at which the creature can use the remaining senses - it now only has 60 ft of extra senses instead of 120 ft.

I could also imagine a case where a creature has darkvision further than its truesight (so the opposite to the Canoloth). In that case, there's a thematic element as well: the creature can see in the dark at long range, but it can only see through illusions if it can get up close and take a good look at them.