Ulamog, the Defiler can arrive already prepared to dismantle a battlefield. Its cast trigger exiles the top half of a targeted opponent's library, rounded up. When Ulamog enters, it receives +1/+1 counters equal to the greatest mana value among cards in exile, and those counters determine the size of its annihilator ability.
The two abilities support each other. Exiling a large portion of a library increases the chance that an expensive card is available to set the counter count. Existing exiled cards also matter, including cards belonging to other players. The number of counters is not determined by how many cards were exiled; one high-mana-value card can be more important than a large pile of cheap spells.
Eldrazi ramp decks can cast it with help from Eldrazi Temple and substantial mana production. Reanimation or Sneak Attack can also put it into play, but those methods miss the library-exiling cast trigger. You still receive the entry counters based on the cards already in exile. Haste is especially valuable because annihilator happens when Ulamog attacks, before blockers are declared.
Ward makes targeted interaction costly by asking the opponent to sacrifice two permanents, but it does not make Ulamog untouchable. Untargeted sweepers and sacrifice effects can still matter. Counter manipulation also changes its annihilator count after it enters. Plan around getting a useful attack, not simply resolving a ten-mana creature: that attack is where the accumulated counters begin to remove opposing resources.
