Well Ι don't have an actual spell in mind, but since Strahd is a 16th level necromancer he may have developed one as he has done with so many others from Nova Arcanum (Forbidden Lore boxed set)
Strahd's Baneful Attractor (Strahd's Baneful Attractor is a spell that makes the spell's target a magnet for nearby spells directed at other nearby creatures (but not area of effect spells)
Strahd's Frightful Joining (The spell allows an undead caster to merge its mind with that of a living victim. For the duration of the spell, the caster lies inert and the caster's mind controls the victim. At the end of the spell, the victim is usually driven mad.)
Strahd's Malefic Meld (allowing the creation of new creatures from up to three component creatures)
Ground Fog
Mimic Mortal This is a spell that Strahd has developed and for it's duration a vampire spellcaster can sacrifice one or more of it's vampiric powers to become more lifelike and immune to vampire weaknesses.
Strahd may have developed a spell similar to a reversed version of the spell mimic mortal, not the feign undead spell. This spell may slowly mimic a vampire transformation taunting the target and this way he creates distress to the party.
it may be similar in effect to the magical coffin in Dominiani's Dark Keep from
Feast of Goblyns.
Misty Summons is a spell created by Strahd and he may use it on the PC's to transfer them to his Castle.
This 7th level spell needs an expensive portal to be build before casting it, the wizard visualizes a place he knows well (Strahd is the Land so he can visualize any place and he owns a crystal ball as well) it must be within the range of the spell 1mile/level and the same domain from the caster. The caster must be within 30 feet from the portal. Over the next 7 rounds a thick mist coalesces that distant place with the portal, when the fog is created the caster does not need to concentrate on it any more and lasts for 10 rounds/level. The fog is magical (alteration and invocation). Any creature stepping into the fog walks out from it through the portal. Anyone weeing the mist can decide not to enter it or walk away if cast on them in 7 rounds. If they are sleeping or immobile they are transported through the portal. It can also be cast on normal fog and make it indistinguishable unless a
detect magic spell is cast on the fog. Strahd has a misty portal build somewhere in his Castle, it;s exact location is uknown (up to the DM to decide), With it he can summon people to his castle anytime. Once there they rarely return.
Then there is the process of making a Vampire Companion, also known as a Vampire Bride or Groom, from VRGtVampires.
It is similar with the process Mina is under in Brahm Stoker's Dracula. In the novel Dracula attacks Mina three times, drinking her blood each time and forcing Mina to drink his blood on the final visit. She is cursed to become a vampire after her death unless Dracula is killed.
You can also change the adventure a bit and have the PC's wife being the reincarnation of Tatyanna.