My suggestion? 1 mass death per deck permitted. 1 chance at a 50/50 situation.
I feel like a more elegant solution would be making the card a Unique Ability like Calamity, maybe adding some flavour text to it that refers to it being a spell that won a famous battle in the lore. Its definitely the spell that I feel most deserves to be placed on the same level as Calamity due to its combination of power and playability.
Players can still include multiple copies to boost their chance of drawing it and Verore tends to run Recycle/Gather Thoughts anyway so they'll often get a chance to cast it again if they save those cards for MD, but it would remove the issue of successfully dodging a turn 6 Mass Death only to get hit with a second one on turn 7. This way dodging Mass Death when they can't get it back into their deck on the same turn actually feels like it properly rewards your prediction as you have an opening to attack all out until they remove it from their discard pile.
Of course some people have said that spammable Mass Death is needed in order to prevent 2-3 purity Verore from simply getting slaughtered by aggro or mass tokens. However for those scenarios you have overcharged storm. When this card came out I initially thought "What the hell is Lightmare thinking letting CoV potentially run SIX board clears?" Then I thought "Maybe they won't include Mass Death in that core set they've got planned, meaning it'll be phased out of the game and this is its replacement." That disappointingly wasn't the case of course, but its still true if MD were to be nerfed Overcharged Storm remains as a potential replacement thats arguably even better at dealing with Flame Dawn rush (can be played a turn earlier and hits paladins) while forcing Verore to either spend lots of cards overcharging or also use their single target spells if they want to get rid of bulky creatures in the more midrange or lategame decks that it feels most unfairly punishing towards.