When you get old and suffer from Alzheimer's or dementia, you lose your memory, your ability to think, and your control over bodily functions. In a way, you become a different person. That's what RELIC tries to explore: the terror of realizing that someone who you've known and loved your entire life has become a stranger.
It's a fairly efficient movie, landing its central metaphor and then GTFO. But what I appreciate most about it is its texture. Felt like very little CG was used. Instead, the director opts for rich production design with locations that feel old and lived in, plus some pretty impressive "creature" effects.
As a debut film, this is a staggering achievement.