What do these Easter Eggs in Van Richten's Guide to Ravenloft refer to?

Question

This article about horror in D&D on the Dungeons & Dragons website mentions the following list of Easter eggs that appear in Van Richten's Guide to Ravenloft:

• [...]
• The mystical symbol formed by the Sigil Lakes of Mordent shape the landscape of more than just that domain. Where else does this symbol appear, and what connection do these mysterious landmarks hold?
• The haunted curio shop depicted in chapter 1 holds several fateful relics, such as an icon of the Lower Aerial Kingdoms (see chapter 4) and a familiar puzzle box (see Baldur’s Gate: Descent into Avernus).
• Supposedly the last member of the Eris family lies buried in the crypts of Castle Ravenloft, but nobles in other domains also claim to be last of the Eris line. Who among them in the true soul heiress?
• Isolde, leader of the Carnival, has ties to more than one group of otherworldly entertainers. What could have become of her former crew?
• If your copy of Van Richten’s Guide to Ravenloft features instructions on page 257, do not follow them under absolutely any circumstances.
• Several creepy tales from the Domains of Dread are among those embodied by the artifact known as the Teeth of Dahlver-Nar (detailed in Tasha’s Cauldron of Everything).
• The closets in area 2 of the House of Lament have an ominous connection to Death House (in Curse of Strahd). Who knows what might happen should the fateful contents within be reunited?

What do these various Easter eggs refer to?
(For instance, where else does the mystical symbol of the Sigil Lakes appear?)

Are they all references to D&D 5e content, or do some refer to content from previous editions?

Answer 1

The mystical symbol formed by the Sigil Lakes of Mordent shape the landscape of more than just that domain. Where else does this symbol appear, and what connection do these mysterious landmarks hold?

This arrangement of three triangles in a triangle can be found in many of the other maps featured in Van Richten's Guide to Ravenloft. The likely ones I spotted are:

• Dementlieu's "Three Odd Gables" buildings are triangular and in the same formation
• in Hazlan, the "Philosopher's Egg" is nestled between three mountains in a triangle
• in Har'Akir, above the "White Oasis" the same shape appears in the terrain
• in Lamordia, the "Old Towers" are three towers built on three hills in a triangle
• in Tepest, "Gwydion's Claw" is a mountain with three peaks in a triangular formation

I didn't find landmarks that fit the pattern in every map, but I'm not assuming I spotted them all. The "Sigil Lakes" description in Mordent's section suggests that some druids were trying to use the area in a ritual to travel to similarly shaped places in other lands, suggesting there is some kind of connection between all these various domains which could be used to travel between them. I'm not aware of any deeper reference the symbol could be invoking.

Answer 2

They're hooks and story prompts.

Van Richten's Guide to Ravenloft (VRGtR) makes references to other material, some of which are:

[...] mysteries we left purposefully vague so you can define them in your adventures.

Explanations for each reference

The haunted curio shop depicted in chapter 1 holds several fateful relics, such as an icon of the Lower Aerial Kingdoms (see chapter 4) and a familiar puzzle box (see Baldur’s Gate: Descent into Avernus).

The puzzle box is an infernal puzzle box from Descent into Avernus. It's shown on the table in the reflection partially obscured by the tiefling.

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Supposedly the last member of the Eris family lies buried in the crypts of Castle Ravenloft, but nobles in other domains also claim to be last of the Eris line. Who among them in the true soul heiress?

This is a link to content is Curse of Strahd which does have a "Tatsaul Eris—Last of the Line" crypt. A number of Eris family members appear in VRGtR.

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Isolde, leader of the Carnival, has ties to more than one group of otherworldly entertainers. What could have become of her former crew?

This is a vague prompt referring to the previous carnival of Isolde that was traded to a pair of shadar-kai.

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If your copy of Van Richten’s Guide to Ravenloft features instructions on page 257, do not follow them under absolutely any circumstances.

This is a joke. There is no page 257; the book has 256 pages.

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Several creepy tales from the Domains of Dread are among those embodied by the artifact known as the Teeth of Dahlver-Nar (detailed in Tasha’s Cauldron of Everything).

• The Staring Cats of Uldun-dar appear as a chance encounter in Hazlan in VRGtR.
• The Mill Road Murders summons three hags which is alluded to as "Mill Road hags" in VRGtR.
• The namesake of "The Thousand Deaths of Jander Sunstar" appears as a mist wanderer in VRGtR.
• Dahlver-Nar, the namesake of the artifact, is also the namesake of a reincarnation mechanic in Curse of Strahd.

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The closets in area 2 of the House of Lament have an ominous connection to Death House (in Curse of Strahd). Who knows what might happen should the fateful contents within be reunited?

Refers to the empty hat box in the House of Lament and a top hat in Death House.