For what is essentially a big block of stone on wheels, the Sacral Vault battle engine is a rather intricate model to build and paint. From what I've seen, many hobbyists forego gluing on the many candles, skulls, and other adornments in the effort to get this thing finished as soon as possible. But for me I couldn't resist piling on the candles.
I also opted to convert the model to change the nature of the sacrifice being conducted on the vault's altar. The original model has a decapitated human tied to the altar, with the Gatorman holding the severed head aloft. I used a Legion of Everblight Spell Martyr to change the sacrifice to a Nyss (a wild elf from the north) with his ribcage burst open. The Gatorman now brandishes the unfortunate Nyss's extracted soul. The "soul" was a leftover swirling soul from another model that I had in my bits box (Lich Lord Asphyxious). The game rules for the Sacral Vault very much revolve around the collection and use of souls, so this conversion reflects this very nicely, in my opinion.
I had some issues selecting colours for this model, as I collect and play all the forces that this battle engine will work for (Minions, Legion of Everblight, Circle Orboros), and so it needs to not look out of place in any of these. I essentially merged the colour palettes and base decorations I use for all three of these armies (although I left off the snow that I would normally use for Legion of Everblight models).