Rules PSA for this potent, yet potentially confusing card:
- Lavinia has two different abilities (one continuous, one triggered) that function in very different ways. Both, however, are only effective if Lavinia is in play before the opponent begins to announce a spell. Flashing in Lavinia with an opposing spell on the stack does not stop that spell in any way.
The first ability looks only at the converted mana cost of the spell being announced (not how much mana will actually be spent when the costs are paid) and the comparison against the number of lands you control is PROBABLY(*) performed after you have chosen modes and alternate/additional costs for the spell, and chosen targets, but before you pay any costs. Some corollaries:
- you can cast split cards, so long as the half you choose to cast meets the CMC requirement.
- you can cast Gush using its alternate casting cost even if you only control five or six lands.
- similarly, you can cast Ancestral Recall even if Gemstone Mine with a single counter on it is your only permanent, etc.
- you must choose a value of X for spells like Repeal which keeps their total CMC at or below the number of lands you control.
If you do not control enough lands, you cannot announce the spell at all (similar to Meddling Mage or Void Winnower).
- you cannot announce the spell even if, due to some convoluted interaction of mana abilities and replacement effects, the number of lands you control would increase during payment of costs, or if Lavinia would leave play during payment of costs.
The first ability affects only noncreature spells. Hollow One is unaffected, as is Phyrexian Metamorph regardless of the permanent that is eventually copied, etc.
The second ability is a triggered ability which triggers on your opponent announcing a spell. In particular, the second ability applies only to spells that your opponent is legally allowed to announce per Lavinia's first ability.
The second ability checks only the amount of mana your opponent(**) actually paid (not the converted mana cost) during announcement of the spell. In particular:
- the ability sees mana spent on additional or alternate costs, cost increases, and cost replacements. A Mox cast under ordinary circumstances triggers Lavinia. A Mox cast under Sphere of Resistance or Trinisphere does not. Gush or Force of Will played using their alternate costs will, under ordinary circumstances, trigger Lavinia.
- under ordinary circumstances, spells cast "without paying their mana cost" (due to Mind's Desire, Spell Queller, Omniscience, etc) trigger Lavinia.
- spells placed directly onto the stack (due to a storm trigger, for instance) are not cast and do not trigger Lavinia.
Lavinia's second ability is an ordinary triggered ability that counters the triggering spell on resolution. In this way the ability functions similarly to Chalice of the Void:
- the triggering spell is placed on the stack under the Lavinia trigger and triggers other abilities (Monastery Mentor, etc) that looks for players casting spells. Players can also target the triggering spell with countermagic, etc.
- Lavinia's second ability can be countered by spells or abilities like Stifle.
- Lavinia's second ability does nothing to uncounterable spells like Abrupt Decay.
Confusingly, Lavinia's second ability triggers on both creature and noncreature spells. So if your opponent controls only Bazaar of Baghdad, they can announce a free Hollow One (despite controlling fewer than five lands) but it will trigger Lavinia's second ability.
(*): That is my interpretation of the current comp. rules. However, this kind of conditional prohibition on spell casting is novel: previous effects like Void Winnower or Gaddock Teeg looked only at the characteristics of the spell, whereas Lavinia looks at both spell characteristics and the board state. So it's possible Lavinia's functionality could change if the spellcasting rules are revised/clarified in a future rules update.
(**): Technically Lavinia looks if any player paid any mana to cast the spell, a distinction that matters only in extreme corner cases.