What are all the ways that stacking resistance becomes immunity by RAW?

Question

In this supplement for Rise of Tiamat, on page 4, the properties of the "dragon mask" magic items are described, including this property (bold emphasis mine):

Damage Absorption. You have resistance against the mask’s damage type. If you already have resistance to that damage type from another source, you instead have immunity to that damage type. If you already have immunity to that damage type from another source, whenever you are subjected to damage of that type, you take none of that damage and regain a number of hit points equal to half the damage dealt of that type.

Off the top of my head, I cannot think of any other feature, be it class feature, racial feature or magic item, that allows multiple forms of resistance to stack and become immunity to either a damage type or a condition. Is this a unique feature of the dragon masks (I'm not so interested in the part about regaining hit points after the part I emphasised, just the part about stacking resistances that I emphasised), or are there are features or magic items RAW that allow stacked resistances to become immunity?

Answer 1

This particular search on DND Beyond reveals 1 racial trait, 2 spells, and 21 magic items having both "resistance" and "immunity" in their descriptions. The only items appearing having the sought after property are the ones already observed: the dragon masks.

Is this a thorough enough search? I would think so. An ability, spell description, or magic item certainly must contain both the words "resistance" and "immunity" (or one of their forms, I tried all those combinations as well), so if it exists, it at least is not on DND Beyond.

Now for the details of my reasoning.

First, we have the rules for damage resistance:

Multiple instances of resistance or vulnerability that affect the same damage type count as only one instance.

And of course, specific beats general:

If a specific rule contradicts a general rule, the specific rule wins.

The general rule is that multiple instances of resistance count as one. For a particular rule to trump this, it must be a specific rule that mentions resistance and immunity. The idea is that for it to be specific for the purposes of satisfying specific-beats-general, it must cite resistance to override normal resistance rules, and it must cite immunity to grant immunity.