In my Opinion, the only question is...how long does Fiendfyre burn? Because, If it does burn indefinitely, then Voldemort might actually be able to kill Alucard. Sure, It would take ages, but in the end Fiendfyre would be able to kill him...
So If Alucard cannot put the Fiendfyre out he is dead. Also...Alucard is only LowGodTier Regen with Schrödinger. Before that, its very inconclusive how strong his Regenerationn is.
Also, you are overseeing that Voldemort could just...you know, run away. Or Aprate Alucard on an Island. Alucard can't cross waters easily and Im sure it would take him quite some time to get back from a lone island in the Sea. That would give Voldemort prep time to think of something.
So I would vote this as inconclusive. Alucard would never be able to catch Voldemort and noone knows if Fiendfyre would kill Alucard. Also, is Help forbidden? Because if Voldemort is allowed to get help… Well, im 100% sure Demetors would kill Alucard. These things EAT Souls, just like Alucard, and they obey Voldemort. A guy with Millions of Souls would be a feast for them and they can only be repelled with the Partonus…also, these things are literally completely immortal.
They are officially stated as being "amortal" in the HPverse, beyond life and death. Untouchable.
Also, does Black Magic work on Alucard like normal? Because Wounds from Black Magic cannot be healed even by Magic…and then there would be poison. Can someone poison Alucard? Basilisk Poison?
This Matchup Is to Inconclusive.
On another note, could people stop to put Hp characters, who physicaly are only superhuman, against People who can move at Hypersonic speeds? This is BS. Ofc they cannot do anything against these dudes. HP characters work by LowScaleRealityManipulation, not by using planetary powers like their foes -.-